io_uring: fix drain alloc fail return code

After a recent change io_drain_req() started to fail requests with
result=0 in case of allocation failure, where it should be and have
been -ENOMEM.

Fixes: 76cc33d7

 ("io_uring: refactor io_req_defer()")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e068110ac4293e0c56cfc4d280d0f22b9303ec08.1625682153.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>