rust: sync: completion: Mark inline complete_all and wait_for_completion

When building the kernel using the llvm-22.1.0-rust-1.93.1-x86_64
toolchain provided by kernel.org with ARCH=x86_64, the following symbols
are generated:

$ nm vmlinux | grep ' _R'.*Completion | rustfilt
ffffffff81827930 T <kernel::sync::completion::Completion>::complete_all
ffffffff81827950 T <kernel::sync::completion::Completion>::wait_for_completion

These Rust methods are thin wrappers around the C completion helpers
`complete_all` and `wait_for_completion`. Mark them `#[inline]` to keep
the wrapper pattern consistent with other small Rust helper methods.

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

Link: https://github.com/Rust-for-Linux/linux/issues/1145
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Fabricio Parra <a@alice0.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Link: https://patch.msgid.link/20260316151056.287-1-a@alice0.com