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: If753b3da3ecb7f974a3c7cea451ebc67e5263e20
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>