io_uring: optimise rw comletion handlers

Don't override req->result in io_complete_rw_iopoll() when it's already
of the same value, we have an if just above it, so move the assignment
there. Also, add one simle unlikely() in __io_complete_rw_common().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8dfeb4f84026a20172bcf82c05010abe955874ae.1634144845.git.asml.silence@gmail.com


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