kernel_netlink: take IPV6_SUBTREES into account.

The linux kernel should be able to deal correctly with source-sensitive ipv6
route entries since 3.11 (when it is configured with IPV6_SUBTREES).  This patch
modify the kernel so that ipv6 route entries sent into the kernel are not pushed
in a different routing table.

The behavior for v4 doesn't change.
Accept to redistribute routes with source address
different than ours.
Don't insert ipv6 disambiguation routes if IPV6_SUBTREES is defined.

Since the kernel behavior is the one desired for ipv6 (match by destination
first, then source), we don't install disambiguation route entries for ipv6.
The behavior for v4 doesn't change.