dm: do not return early from dm_io_complete if BLK_STS_AGAIN without polling

Commit 52919840 ("dm: fix bio polling to handle possibile
BLK_STS_AGAIN") inadvertently introduced an early return from
dm_io_complete() without first queueing the bio to DM if BLK_STS_AGAIN
occurs and bio-polling is _not_ being used.

Fix this by only returning early from dm_io_complete() if the bio has
first been properly queued to DM. Otherwise, the bio will never finish
via bio_endio.

Fixes: 52919840 ("dm: fix bio polling to handle possibile BLK_STS_AGAIN")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@kernel.org>