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 <[e-mail]>
Cc: Jan Kara <[e-mail]>
Acked-by: Cyrill Gorcunov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/udf/balloc.c

···
 {
 	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;