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>