btrfs: deprecate BTRFS_IOC_BALANCE ioctl

The v2 balance ioctl has been introduced more than 9 years ago. Users of
the old v1 ioctl should have long been migrated to it. It's time we
deprecate it and eventually remove it.

The only known user is in btrfs-progs that tries v1 as a fallback in
case v2 is not supported. This is not necessary anymore.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3986,6 +3986,10 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
 	bool need_unlock; /* for mut. excl. ops lock */
 	int ret;
 
+	if (!arg)
+		btrfs_warn(fs_info,
+	"IOC_BALANCE ioctl (v1) is deprecated and will be removed in kernel 5.18");
+
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;