io_uring: remove obsolete @mm_fault

If io_submit_sqes() can't grab an mm, it fails and exits right away.
There is no need to track the fact of the failure. Remove @mm_fault.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>