dm: missing break in process_queued_bios()

his used to be a fall through case, but we shifted code around and I
think we want a break here now.

Fixes: 4e4cbee9 ("block: switch bios to blk_status_t")

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>