io_uring: cleanup personalities under uring_lock

personality_idr is usually synchronised by uring_lock, the exception
would be removing personalities in io_ring_ctx_wait_and_kill(), which
is legit as refs are killed by that point but still would be more
resilient to do it under the lock.

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