block: fix infinite loop if the device loses discard capability
If __blkdev_issue_discard is in progress and a device mapper device is
reloaded with a table that doesn't support discard,
q->limits.max_discard_sectors is set to zero. This results in infinite
loop in __blkdev_issue_discard.
This patch checks if max_discard_sectors is zero and aborts with
-EOPNOTSUPP.