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.

Fixes: e68d496ada ("LU-15858 sec: reinstate null encryption for file names")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ibe52feb670a00c9f421907ecd438bcccc62856f0