rust: sync: Mark PollCondVar::drop() inline

When building the kernel using the llvm-18.1.3-rust-1.85.0-x86_64
with ARCH=arm64, the following symbols are generated:

$nm vmlinux | grep ' _R'.*PollCondVar  | rustfilt
... T <kernel::sync::poll::PollCondVar as kernel::init::PinnedDrop>::drop
...

This Rust symbol is trivial wrappers around the C functions
__wake_up_pollfree() and synchronize_rcu(). It doesn't make sense to go
through a trivial wrapper for its functions, so mark it inline.

[boqun: Reword the commit title and re-format the commit log per tip
tree's requirement, remove unnecessary information from "nm vmlinux"
result.]

Link: https://github.com/Rust-for-Linux/linux/issues/1145


Suggested-by: Alice Ryhl <aliceryhl@google.com>
Co-developed-by: Grace Deng <Grace.Deng006@Gmail.com>
Signed-off-by: Grace Deng <Grace.Deng006@Gmail.com>
Signed-off-by: Kunwu Chan <kunwu.chan@hotmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250317025205.2366518-1-kunwu.chan@linux.dev