dm table: avoid crash if integrity profile changes

Commit a63a5cf8 (dm: improve block integrity support) introduced a
two-phase initialization of a DM device's integrity profile.  This
patch avoids dereferencing a NULL 'template_disk' pointer in
blk_integrity_register() if there is an integrity profile mismatch in
dm_table_set_integrity().

This can occur if the integrity profiles for stacked devices in a DM
table are changed between the call to dm_table_prealloc_integrity() and
dm_table_set_integrity().
Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: stable@kernel.org # 2.6.39

drivers/md/dm-table.c

@@ -1238,14 +1238,15 @@ static void dm_table_set_integrity(struct dm_table *t)
 		return;
 
 	template_disk = dm_table_get_integrity_disk(t, true);
-	if (!template_disk &&
-	    blk_integrity_is_initialized(dm_disk(t->md))) {
-		DMWARN("%s: device no longer has a valid integrity profile",
-		       dm_device_name(t->md));
-		return;
-	}
+	if (template_disk)
 		blk_integrity_register(dm_disk(t->md),
 				       blk_get_integrity(template_disk));
+	else if (blk_integrity_is_initialized(dm_disk(t->md)))
+		DMWARN("%s: device no longer has a valid integrity profile",
+		       dm_device_name(t->md));
+	else
+		DMWARN("%s: unable to establish an integrity profile",
+		       dm_device_name(t->md));
 }
 
 static int device_flush_capable(struct dm_target *ti, struct dm_dev *dev,