fs/ext4/ext4fs.c: Free dirnode in error path of ext4fs_ls

As reported by Coverity, we did not free dirnode in the case of failure.
Do so now.

Reported-by: Coverity (CID: 131221)
Cc: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Tom Rini <trini@konsulko.com>