block: export blk_rq_is_poll

This is in preparation to support iopoll for nvme passthrough.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Link: https://lore.kernel.org/r/20220823161443.49436-4-joshi.k@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>