io_uring: check if opcode needs poll first on arming

->pollout or ->pollin are set only for opcodes that need a file, so if
io_arm_poll_handler() tests them first we can be sure that the request
has file set and the ->file check can be removed.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9adfe4f543d984875e516fce6da35348aab48668.1634987320.git.asml.silence@gmail.com


Reviewed-by: Hao Xu <haoxu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>