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

vm-migration/src/protocol.rs

@@ -87,9 +87,8 @@
 //! to newer ones.
 
 use std::io::{Read, Write};
-use std::mem::size_of;
 use std::ops::RangeInclusive;
-use std::{mem, slice};
+use std::slice;
 
 use anyhow::anyhow;
 use itertools::Itertools;
@@ -577,7 +576,7 @@ impl MemoryRangeTable {
     }
 
     pub fn length(&self) -> u64 {
-        (mem::size_of::<MemoryRange>() * self.data.len()) as u64
+        (size_of::<MemoryRange>() * self.data.len()) as u64
     }
 
     pub fn write_to(&self, fd: &mut dyn Write) -> Result<(), MigratableError> {