io_uring: replace sqd rw_semaphore with mutex

The only user of read-locking of sqd->rw_lock is sq_thread itself, which
is by definition alone, so we don't really need rw_semaphore, but mutex
will do. Replace it with a mutex, and kill read-to-write upgrading and
extra task_work handling in io_sq_thread().

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