LU-15911 enc: null encrypted names is embedded llcrypt only

LU-15911 enc: null encrypted names is embedded llcrypt only

enable_filename_encryption tunable only makes sense when Lustre client
is built against embedded llcrypt. When built against in-kernel
fscrypt, this tunable is silently ignored, as fscrypt always carries
out file name encryption.

So have the enable_filename_encryption tunable only when Lustre client
is built against embedded llcrypt. Also fix sanity-sec test_54 so that
it works for in-kernel fscrypt.

Lustre-change: https://review.whamcloud.com/47520
Lustre-commit: TBD (98621cb3f33e39c458ef1c02da3b23a69d481b61)

Fixes: e68d496ada ("LU-15858 sec: reinstate null encryption for file names")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ibe52feb670a00c9f421907ecd438bcccc62856f0
Reviewed-on: https://review.whamcloud.com/47785
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>