kernel_netlink: fix bug introduced by bad simplification of the code.

The problem was that babeld didn't redistribute default v4
routes anymore. This was due to the fact that if there is no
RTA_DST TLV in a netlink message, then it's a default route.

This patch revert some changes made in
    8f7c6b1 kernel_netlink: take IPV6_SUBTREES into account.