LU-16012 sec: fix detection of SELinux enforcement

LU-16012 sec: fix detection of SELinux enforcement

On newer distros (e.g. RHEL 9.0), on which selinux_is_enabled() does
not exist anymore, the only way to find out if SELinux is enforced
when initializing the security context is to fetch the length of the
security attribute name. If it is 0, we conclude SELinux is disabled.

Lustre-change: https://review.whamcloud.com/48049
Lustre-commit: 155cbc22ba4f758cf9eec415f36f940ca2b23de9

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ifcdcb8ffbb7f9ad50d16d7d3317e94d0d212fa42
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/48193
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>