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

hypervisor/src/kvm/aarch64/mod.rs

@@ -53,7 +53,7 @@ macro_rules! arm64_core_reg_id {
         KVM_REG_ARM64 as u64
             | u64::from(KVM_REG_ARM_CORE)
             | $size
-            | (($offset / mem::size_of::<u32>()) as u64)
+            | (($offset / size_of::<u32>()) as u64)
     };
 }