io_uring/rsrc: extract SCM file put helper

SCM file accounting is a slow path and is only used for UNIX files.
Extract a helper out of io_rsrc_file_put() that does the SCM
unaccounting.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/58cc7bffc2ee96bec8c2b89274a51febcbfa5556.1681210788.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>