[BugFix] [RHEL/6] Fix sysctl_net_ipv6_conf_default_accept_ra false positive

When IPv6 support is disabled, don't perform the runtime sysctl configuration check (since
in that case the required flag won't be set => the check will fail even when it should pass).

This aligns the implementation with current implementation of the OVAL check for other IPv6 net
sysctl_net_ipv6_conf_default_accept_redirects option which performs the scan correctly.

Tested on both 32-bit & 64-bit versions of RHEL-6 & works correctly.

Please review.

Thank you, Jan