dm thin metadata: fix a race when entering fail mode

In dm_thin_find_block() the ->fail_io flag was checked outside the
metadata device's root_lock, causing dm_thin_find_block() to race with
the setting of this flag.

Signed-off-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-thin-metadata.c

···
 	dm_block_t keys[2] = { td->id, block };
 	struct dm_btree_info *info;
 
-	if (pmd->fail_io)
-		return -EINVAL;
-
 	down_read(&pmd->root_lock);
+	if (pmd->fail_io) {
+		up_read(&pmd->root_lock);
+		return -EINVAL;
+	}
 
 	if (can_issue_io) {
 		info = &pmd->info;