dm table: allow targets without devices to set ->io_hints

In dm_calculate_queue_limits, add call to ->io_hints hook if the
target doesn't provide ->iterate_devices.

This is needed so the "error" and "zero" targets may support
discards. The 2 following commits will add their respective discard
support.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-table.c

@@ -1670,8 +1670,12 @@ int dm_calculate_queue_limits(struct dm_table *t,
 
 		blk_set_stacking_limits(&ti_limits);
 
-		if (!ti->type->iterate_devices)
+		if (!ti->type->iterate_devices) {
+			/* Set I/O hints portion of queue limits */
+			if (ti->type->io_hints)
+				ti->type->io_hints(ti, &ti_limits);
 			goto combine_limits;
+		}
 
 		/*
 		 * Combine queue limits of all the devices this target uses.