io_uring: use right helpers for file assign locking

We have io_ring_submit_[un]lock() functions helping us with conditional
->uring_lock locking, use them in io_file_get_fixed() instead of hand
coding.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c9c9ff1e046f6eb68da0a251962a697f8a2275fa.1650311386.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>