ext4: check return of ext4_orphan_get properly

This patch fix a panic while running fsfuzzer. 
We are improperly checking the return of ext4_orphan_get.
Signed-off-by: Josef Bacik <jbacik@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -1594,8 +1594,8 @@ static void ext4_orphan_cleanup (struct super_block * sb,
 	while (es->s_last_orphan) {
 		struct inode *inode;
 
-		if (!(inode =
-		      ext4_orphan_get(sb, le32_to_cpu(es->s_last_orphan)))) {
+		inode = ext4_orphan_get(sb, le32_to_cpu(es->s_last_orphan));
+		if (IS_ERR(inode)) {
 			es->s_last_orphan = 0;
 			break;
 		}