misc: use prelude size_of

size_of is part of std::prelude as of Rust 1.80 (with size_of_val,
align_of, align_of_val), and the workspace MSRV is 1.89, so qualifying
it (mem::size_of, std::mem::size_of, core::mem::size_of) is unnecessary.

Convert every qualified size_of call-site to the bare prelude form and
drop the now-redundant `use std::mem::size_of;` imports, keeping
`use std::mem;` where it still serves non-prelude items (transmute,
swap, replace, take, zeroed, MaybeUninit, offset_of). size_of is the
only one of the four currently used in the tree.

Pure refactor, no behavioural change. Follow-up to the
clippy::absolute_paths cleanup (#7670), as discussed in #8444.

Signed-off-by: Henry Hrvoje Tonkovac <htonkovac@gmail.com>
Assisted-by: Claude:Opus-4.8

virtio-devices/src/block.rs

@@ -666,8 +666,7 @@ impl BlockEpollHandler {
         if let Some(cpuset) = cpuset.as_ref() {
             let cpuset: *const libc::cpu_set_t = cpuset;
             // SAFETY: FFI call with correct arguments
-            let ret =
-                unsafe { libc::sched_setaffinity(0, mem::size_of::<libc::cpu_set_t>(), cpuset) };
+            let ret = unsafe { libc::sched_setaffinity(0, size_of::<libc::cpu_set_t>(), cpuset) };
 
             if ret != 0 {
                 error!(
@@ -1148,7 +1147,7 @@ impl VirtioDevice for Block {
         // The "writeback" field is the only mutable field
         let writeback_offset =
             (&raw const self.config.writeback as u64) - (&raw const self.config as u64);
-        if offset != writeback_offset || data.len() != mem::size_of_val(&self.config.writeback) {
+        if offset != writeback_offset || data.len() != size_of_val(&self.config.writeback) {
             error!(
                 "Attempt to write to read-only field: offset {:x} length {}",
                 offset,