btrfs: replace BUG in can_modify_feature

We don't need to crash hard here, it's just reading a sysfs file. The
values considered in switch are from a fixed set, the default case
should not happen at all.
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/sysfs.c

@@ -75,7 +75,9 @@ static int can_modify_feature(struct btrfs_feature_attr *fa)
 		clear = BTRFS_FEATURE_INCOMPAT_SAFE_CLEAR;
 		break;
 	default:
-		BUG();
+		printk(KERN_WARNING "btrfs: sysfs: unknown feature set %d\n",
+				fa->feature_set);
+		return 0;
 	}
 
 	if (set & fa->feature_bit)