io_uring: do not run task work at the start of io_uring_enter

This is not needed, and it is normally better to wait for task work until
after submissions. This will allow greater batching if either work arrives
in the meanwhile, or if the submissions cause task work to be queued up.

For SQPOLL this also no longer runs task work, but this is handled inside
the SQPOLL loop anyway.

For IOPOLL io_iopoll_check will run task work anyway

And otherwise io_cqring_wait will run task work

Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220830125013.570060-4-dylany@fb.com


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