LU-15922 sec: new connect flag for name encryption

LU-15922 sec: new connect flag for name encryption

Introduce OBD_CONNECT2_ENCRYPT_NAME connection flag for compatibility
with older versions that do not support name encryption.
When server side does not have this flag, client side is forced to
null encryption for file names. And client needs to use old xattr to
store encryption context.

Also update tests in sanity-sec to exercise name encryption only if
server side supports it.

Lustre-change: https://review.whamcloud.com/47574
Lustre-commit: 71d63602c57e6d0cb723ae02ea767c6c170e5997

Fixes: ed4a625d88 ("LU-13717 sec: filename encryption - digest support")
Test-Parameters: serverversion=2.14 testlist=sanity-sec mdscount=2 mdtcount=4 osscount=1 ostcount=8 clientcount=2 fstype=ldiskfs
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I446a4caba8e45821d701628a14c96f03cb6c4525
Reviewed-on: https://review.whamcloud.com/47852
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Oleg Drokin <green@whamcloud.com>