Fix five clippy errors that only appear on the target CI lints

CI's clippy job runs on ubuntu-latest, so cfg(target_os = "linux") code is
what the gate actually compiles — and none of it is built by clippy on a
Mac. Five `-D warnings` errors were sitting in drive/linux.rs,
scsi/linux.rs, io/writeback/linux.rs and the Linux arm of drive/mod.rs:
four collapsible let-chains and one manual `% n == 0`. CI was red on the
lint job while the local gate reported all green.

Collapsed into let-chains, which the declared toolchain supports, and
folded drive/mod.rs's length precondition into its chain so the body no
longer needs a nested block.

Found because an agent working on the SCSI backends reported the lints in
passing while checking that a Windows-only file compiled. Worth noting how
it stayed hidden: every one of these files is cfg-gated to a platform this
machine is not, so no amount of local gating would have surfaced them. The
companion change to the precommit script closes that hole.

src/drive/linux.rs

@@ -23,16 +23,14 @@ pub fn find_drives() -> Vec<(String, DriveId)> {
         if !std::path::Path::new(&path).exists() {
             continue;
         }
-        if let Ok(mut transport) = crate::scsi::open(std::path::Path::new(&path)) {
-            if let Ok(id) = DriveId::from_drive(transport.as_mut()) {
-                if !id.raw_inquiry.is_empty()
+        if let Ok(mut transport) = crate::scsi::open(std::path::Path::new(&path))
+            && let Ok(id) = DriveId::from_drive(transport.as_mut())
+            && !id.raw_inquiry.is_empty()
             && (id.raw_inquiry[0] & 0x1F) == SCSI_PERIPHERAL_TYPE_OPTICAL
         {
             drives.push((path, id));
         }
     }
-        }
-    }
     drives
 }
 

src/drive/mod.rs

@@ -902,21 +902,17 @@ impl Drive {
                 // 2026-05-08) dd via /dev/sr0 recovers ~50% of bad
                 // sectors that a single-shot SG_IO READ misses.
                 #[cfg(target_os = "linux")]
-                if recovery {
-                    if let Some(fd) = self.block_dev_fd {
+                if recovery
+                    && let Some(fd) = self.block_dev_fd
+                    && buf.len() >= count as usize * 2048
+                {
                     let len = count as usize * 2048;
-                        if buf.len() >= len {
                     let offset = lba as i64 * 2048;
                     // Drop kernel cache for this region so we get
                     // a fresh device read, not stale page-cache
                     // data from a prior successful neighbour read.
                     let _ = unsafe {
-                                libc::posix_fadvise(
-                                    fd,
-                                    offset,
-                                    len as i64,
-                                    libc::POSIX_FADV_DONTNEED,
-                                )
+                        libc::posix_fadvise(fd, offset, len as i64, libc::POSIX_FADV_DONTNEED)
                     };
                     let n = unsafe {
                         libc::pread(fd, buf.as_mut_ptr() as *mut libc::c_void, len, offset)
@@ -940,8 +936,6 @@ impl Drive {
                         "/dev/sr0 pread fallback also failed"
                     );
                 }
-                    }
-                }
 
                 Err(Error::DiscRead {
                     sector: lba as u64,

src/io/writeback/linux.rs

@@ -272,7 +272,7 @@ impl WritebackPipeline {
             self.chunk_bytes,
             self.skip_wait(),
         );
-        if self.chunk_count % SIZE_LOG_INTERVAL == 0 {
+        if self.chunk_count.is_multiple_of(SIZE_LOG_INTERVAL) {
             tracing::debug!(
                 target: "mux",
                 "WritebackPipeline chunk_bytes={} after {} chunks is_nfs={} degraded={}",

src/scsi/linux.rs

@@ -169,13 +169,13 @@ impl SgIoTransport {
 
         if dev_name.starts_with("sr") {
             let sg_dir = format!("/sys/class/block/{}/device/scsi_generic", dev_name);
-            if let Ok(mut entries) = std::fs::read_dir(&sg_dir) {
-                if let Some(Ok(entry)) = entries.next() {
+            if let Ok(mut entries) = std::fs::read_dir(&sg_dir)
+                && let Some(Ok(entry)) = entries.next()
+            {
                 let sg_name = entry.file_name();
                 return std::path::PathBuf::from(format!("/dev/{}", sg_name.to_string_lossy()));
             }
         }
-        }
 
         device.to_path_buf()
     }