LU-15858 sec: reinstate null encryption for file names

LU-15858 sec: reinstate null encryption for file names

Reinstate null encryption for file names by adding a new llite
parameter named 'enable_filename_encryption', set to 0 by default.
When this parameter is 0, new empty directories configured as
encrypted ignore the filenames_encryption_mode and use
LLCRYPT_MODE_NULL instead, which is a no-op. This LLCRYPT_MODE_NULL
mode is inherited for all subdirectories and files.
When this parameter is 1, new empty directories configured as
encrypted use the normal encryption mode.

To set this parameter globally for all clients, do on the MGS:
mgs# lctl set_param -P llite.*.enable_filename_encryption=0

Also update sanity-sec test_54 to exercise the new parameter
'enable_filename_encryption'.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I9d726ba26cc91a51690d59a81efe3eb98ee2995c
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/47355
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>