Expanded testing of DM's bio polling support (using more fio threads
to dm-linear ontop of null_blk) exposed the possibility for polled
bios to hang (repeatedly polling in io_uring) when null_blk responds
with BLK_STS_AGAIN (due to lack of resources):

1) io_complete_rw_iopoll() is called from blkdev_bio_end_io_async() to
   notify kiocb is done, that is the completion interface between block
   layer and io_uring

2) io_complete_rw_iopoll() is called from io_do_iopoll()

3) dm returns BLK_STS_AGAIN for one bio (on behalf of underlying
   driver), then io_complete_rw_iopoll is called, but io_do_iopoll()
   doesn't handle -EAGAIN at all (due to logic in io_rw_should_reissue)

4) reason for dm's BLK_STS_AGAIN is underlying null_blk driver ran out
   of requests (easier to reproduce by setting low hw_queue_depth).

5) dm should handle BLK_STS_AGAIN for POLLED underlying IO, and may
   retry in dm layer.

This fix adds REQ_POLLED specific BLK_STS_AGAIN handling to
dm_io_complete() that clears REQ_POLLED and requeues the bio to DM
using queue_io().

Fixes: b99fdcdc ("dm: support bio polling")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
[snitzer: revised header, reused dm_io_complete's REQ_POLLED case]
Signed-off-by: Mike Snitzer <snitzer@kernel.org>