rust: file: optimize rust symbol generation for FileDescriptorReservation

rust: file: optimize rust symbol generation for FileDescriptorReservation

When build 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'.*FileDescriptorReservation | rustfilt
... T <kernel::fs::file::FileDescriptorReservation>::fd_install
... T <kernel::fs::file::FileDescriptorReservation>::get_unused_fd_flags
... T <kernel::fs::file::FileDescriptorReservation as core::ops::drop::Drop>::drop

These Rust symbols are trivial wrappers around the C functions
fd_install, put_unused_fd and put_task_struct. It
doesn't make sense to go through a trivial wrapper for these
functions, so mark them inline.

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>
Link: https://lore.kernel.org/r/20250317023702.2360726-1-kunwu.chan@linux.dev
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>