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/scsi/linux.rs

@@ -169,11 +169,11 @@ 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() {
-                    let sg_name = entry.file_name();
-                    return std::path::PathBuf::from(format!("/dev/{}", sg_name.to_string_lossy()));
-                }
+            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()));
             }
         }