io_uring: refactor io_openat2()

Put do_filp_open() fail path of io_openat2() under a single if,
deduplicating put_unused_fd(), making it look better and helping
the hot path.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f4c84d25c049d0af2adc19c703bbfef607200209.1624543113.git.asml.silence@gmail.com


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