network: selinux hook handling to enumerate nexthop

network: 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.