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/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={}",