io_uring: optimise submission loop invariant

Instead of keeping @submitted in io_submit_sqes(), which for each
iteration requires comparison with the initial number of SQEs, store the
number of SQEs left to submit. We'll need nr only for when we're done
with SQE handling.

note: if we can't allocate a req for the first SQE we always has been
returning -EAGAIN to the userspace, save this behaviour by looking into
the cache in a slow path.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c3b3df9aeae4c2f7a53fd8386385742e4e261e77.1649771823.git.asml.silence@gmail.com


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