io_uring/uring_cmd: ensure that device supports IOPOLL

It's possible for a file type to support uring commands, but not
pollable ones. Hence before issuing one of those, we should check
that it is supported and error out upfront if it isn't.

Cc: stable@vger.kernel.org
Fixes: 5756a3a7 ("io_uring: add iopoll infrastructure for io_uring_cmd")
Link: https://github.com/axboe/liburing/issues/816


Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>