Btrfs: add basic infrastructure for selective balancing

This allows to have a separate set of filters for each chunk type
(data,meta,sys).  The code however is generic and switch on chunk type
is only done once.

This commit also adds a type filter: it allows to balance for example
meta and system chunks w/o touching data ones.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/btrfs/ioctl.c

@@ -3116,6 +3116,9 @@ static long btrfs_ioctl_balance(struct btrfs_root *root, void __user *arg)
 		memcpy(&bctl->sys, &bargs->sys, sizeof(bctl->sys));
 
 		bctl->flags = bargs->flags;
+	} else {
+		/* balance everything - no filters */
+		bctl->flags |= BTRFS_BALANCE_TYPE_MASK;
 	}
 
 	ret = btrfs_balance(bctl, bargs);