ext4 crypto: release crypto resource on module exit

Crypto resource should be released when ext4 module exits, otherwise
it will cause memory leak.

Change-Id: Ie298e73bd766768707a7af440691ce2f418f5acc
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>