ocfs2: fix possible double free in ocfs2_reflink_xattr_rec

In ocfs2_reflink_xattr_rec(), meta_ac and data_ac are allocated by calling
ocfs2_lock_reflink_xattr_rec_allocators().

Once an error occurs when allocating *data_ac, it frees *meta_ac which is
allocated before.  Here it mistakenly sets meta_ac to NULL but *meta_ac.
Then ocfs2_reflink_xattr_rec() will try to free meta_ac again which is
already invalid.
Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ocfs2/xattr.c

@@ -6802,7 +6802,7 @@ static int ocfs2_lock_reflink_xattr_rec_allocators(
 	if (ret) {
 		if (*meta_ac) {
 			ocfs2_free_alloc_context(*meta_ac);
-			meta_ac = NULL;
+			*meta_ac = NULL;
 		}
 	}