udf: fix udf_add_free_space

In commit 742ba02a

 (udf: create common
function for changing free space counter) by accident I reversed safety
condition which lead to null pointer dereference in case of media error and
wrong counting of free space in normal situation

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>