LU-16045 enc: force use of new enc xattr on new servers

LU-16045 enc: force use of new enc xattr on new servers

When an older client uses encryption with a newer server, the client
wants to see the encryption context in security.c xattr. But
internally on server side, we force use of newer encryption.c xattr
for consistency purpose. When required, the encryption context is put
in the request to the client as usual, which interprets it as desired.

Fixes: ba50f9a977 ("LU-13717 sec: make client encryption compatible with ext4")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I667e123bdff912acc270666e8c74ebda6f0534e7
Reviewed-on: https://review.whamcloud.com/48034
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>