io_uring: simplify fallback execution

Lock the ring with uring_lock in io_fallback_req_func(), which should
make it a bit safer and easier. With that we also don't need refs
pinning as io_ring_exit_work() will wait until uring_lock is freed.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/56170e6a0cbfc8edee2794c6613e8f6f1d76d276.1673887636.git.asml.silence@gmail.com


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