btrfs: rename err to ret in btrfs_ioctl_snap_destroy()

Unify naming of return value to the preferred way.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>