io_uring: don't reinit submit state every time

As now submit_state is retained across syscalls, we can save ourself
from initialising it from ground up for each io_submit_sqes(). Set some
fields during ctx allocation, and just keep them always consistent.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
[axboe: remove unnecessary zeroing of ctx members]
Signed-off-by: Jens Axboe <axboe@kernel.dk>