btrfs: device add must be sysloged

when we add a new disk to the mounted btrfs we don't record it
as of now, disk add is a critical change of btrfs configuration,
it must be recorded in the syslog to help offline investigations
of customer problems when reported.
Signed-off-by: Anand Jain <Anand.Jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ioctl.c

@@ -2633,6 +2633,9 @@ static long btrfs_ioctl_add_dev(struct btrfs_root *root, void __user *arg)
 	vol_args->name[BTRFS_PATH_NAME_MAX] = '\0';
 	ret = btrfs_init_new_device(root, vol_args->name);
 
+	if (!ret)
+		btrfs_info(root->fs_info, "disk added %s",vol_args->name);
+
 	kfree(vol_args);
 out:
 	mutex_unlock(&root->fs_info->volume_mutex);