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>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>
Change-Id: I741ae219615188d9ddd395f8e71baae05f64fef4