LU-14479 ssk: explicitly set perm on key

LU-14479 ssk: explicitly set perm on key

When an SSK key is loaded, either via lgss_sk command or thanks to
skpath mount option, try to set permissions on the key.
This is to avoid a 'Permission denied' error when a Lustre client or
server wants to make use of the key later on.

Lustre-change: https://review.whamcloud.com/41929
Lustre-commit: f265033840996dcdffb2f05a64b51b51391a273c

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1ed712ae4d07be306cc76b4e59fab303437558bb
Reviewed-on: https://review.whamcloud.com/43164
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>