udf: fix udf_add_free_space

In commit 742ba02a51c8d0bf5446b154531179760c1ed0a2 (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>

fs/udf/balloc.c

@@ -145,7 +145,7 @@ static bool udf_add_free_space(struct udf_sb_info *sbi,
 {
 	struct logicalVolIntegrityDesc *lvid;
 
-	if (sbi->s_lvid_bh)
+	if (sbi->s_lvid_bh == NULL)
 		return false;
 
 	lvid = (struct logicalVolIntegrityDesc *)sbi->s_lvid_bh->b_data;