LU-12275 sec: introduce null algo for filename encryption

LU-12275 sec: introduce null algo for filename encryption

Introduce a "null" algorithm for client side filename encryption,
which is basically a no-op.
This is needed because first implementation step only supports
content encryption, and not names. So give the ability to support
encryption policies that have a 'filenames_encryption_mode' property
internally set to LLCRYPT_MODE_NULL.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I3470f89f227b3a03e56766fe1ba5e36ae92ec27b
Reviewed-on: https://review.whamcloud.com/38881
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>