The purpose of the this new filter is to override *locally*:
- RTA_SRC ('from' option in ip-route)
- RTA_PREFSRC ('src' in ip-route)

We also force use of ipv6 subtrees because:
- even Linux 2.6.32 has them
- the fallback implementation using a separate table is not equivalent,
  at least not for Nexedi (and we don't need RTA_SRC for ipv4)

TODO: documentation

To be pushed upstream.

To be pushed upstream.
Co-authored-by: Julien Muchembled <jm@nexedi.com>

To be pushed upstream.
Co-authored-by: Julien Muchembled <jm@nexedi.com>

To be pushed upstream.

This reverts commit 9edf0a14.

Thanks to Pierre Pfister.

When restarting network on openwrt, the rules were flushed.  In
this patch, we ask the kernel for rule changes, and try to
reinstall (or flush) the rules, as it should be.

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.

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.