f2fs crypto: get rid of ci_mode from struct f2fs_crypt_info

This patch integrates the below patch into f2fs.

"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."

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>