io_uring: pin ctx on fallback execution

Pin ring in io_fallback_req_func() by briefly elevating ctx->refs in
case any task_work handler touches ctx after releasing a request.

Fixes: 9011bf9a ("io_uring: fix stuck fallback reqs")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/833a494713d235ec144284a9bbfe418df4f6b61c.1629235576.git.asml.silence@gmail.com


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