io_uring: pre-calculate syscall iopolling decision

Syscall should only iopoll for events when it's a IOPOLL ring and is not
SQPOLL. Instead of check both flags every time we can save it in ring
flags so it's easier to use. We don't care much about an extra if there,
however it will be inconvenient to copy-paste this chunk with checks in
future patches.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7fd2f8fc2606305aa06dd8c0ff8f76a66b39c383.1647957378.git.asml.silence@gmail.com


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