Btrfs: Check metadata redundancy on balance

When converting a filesystem via balance check that metadata mode
is at least as redundant as the data mode. For example give warning
when:
-dconvert=raid1 -mconvert=single
Signed-off-by: NSam Tygier <samtygier@yahoo.co.uk>
[ minor message reformatting ]
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -3748,6 +3748,13 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 		}
 	} while (read_seqretry(&fs_info->profiles_lock, seq));
 
+	if (btrfs_get_num_tolerated_disk_barrier_failures(bctl->meta.target) <
+		btrfs_get_num_tolerated_disk_barrier_failures(bctl->data.target)) {
+		btrfs_warn(fs_info,
+	"metatdata profile 0x%llx has lower redundancy than data profile 0x%llx",
+			bctl->meta.target, bctl->data.target);
+	}
+
 	if (bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) {
 		fs_info->num_tolerated_disk_barrier_failures = min(
 			btrfs_calc_num_tolerated_disk_barrier_failures(fs_info),