systemd: selinux hook handling to enumerate nexthop

When selinux is enabled, the call of
manager_rtnl_enumerate_nexthop() fails.
This fix is to facilitate selinux hook handling
for enumerating nexthop.

In manager_rtnl_enumerate_nexthop() there is a check
if "Not supported" is returned by the send_netlink() call.
This check expects that -EOPNOTSUPP is returned,
the selinux hook seems to return -EINVAL instead.
This happens in kernel older than 5.3
(more specificallytorvalds/linux@65ee00a) as it does not support
nexthop handling through netlink.
And if SELinux is enforced in the order kernel, callingRTM_GETNEXTHOP
returns -EINVAL.

Thus adding a call in the manager_rtnl_enumerate_nexthop for the
extra return -EINVAL.

Note: systemd version is different in yocto project (v246.6) and
systemd master(v247) and In systemd verison(246.6)
mac_selinux_enforcing() function is not declared and defined.

Signed-off-by: Purushottam choudhary <purushottam.choudhary@kpit.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>