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.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ifcdcb8ffbb7f9ad50d16d7d3317e94d0d212fa42
Reviewed-on: https://review.whamcloud.com/48049
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>