rust: kernel: cpu: mark `CpuId::current()` inline

rust: kernel: cpu: mark `CpuId::current()` inline

When building the kernel using llvm-20.1.7-rust-1.89.0-x86_64,
this symbol is generated:

$ llvm-nm --demangle vmlinux | grep CpuId
ffffffff84c77450 T <kernel::cpu::CpuId>::current

However, this Rust symbol is a trivial wrapper around
`raw_smp_processor_id` function. It doesn't make sense
to go through a trivial wrapper for such functions,
so mark it inline.

After applying this patch, the above command will produce no output.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://github.com/Rust-for-Linux/linux/issues/1145
Signed-off-by: Ritvik Gupta <ritvikfoss@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>