fs: add file_operations->uring_cmd_iopoll

io_uring will invoke this to do completion polling on uring-cmd
operations.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Link: https://lore.kernel.org/r/20220823161443.49436-2-joshi.k@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>