ext4 crypto: exit cleanly if ext4_derive_key_aes() fails

Return value of ext4_derive_key_aes() is stored but not used.
Add test to exit cleanly if ext4_derive_key_aes() fail.
Also fix coverity CID 1309760.

Signed-off-by: Laurent Navet <laurent.navet@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>