ext4 crypto: get rid of ci_mode from struct ext4_crypt_info

The ci_mode field was superfluous, and getting rid of it gets rid of
an unused hole in the structure.

Change-Id: I0f4c38a1162fa9c6da8a3529b7477ff5560c21df
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>