io_uring: prioritise read success path over fails

Rearrange io_read return handling so first we expect it completing
successfully and only then checking for errors, which is a colder path.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c91c7c2da11815ec8b04b5d872f60dc4cde662c5.1634144845.git.asml.silence@gmail.com


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