route cache: Fix handling of ipv6 multipath routes

route cache: Fix handling of ipv6 multipath routes

There are two ways kernel handles ipv6 equal cost multipath routes
depending on which kernel version you are looking at.

older kernels without ipv6 ECMP support, accept the below ECMP routes,
#ip -6 route add 2001::/16 nexthop via fe80:2::2 dev swp1
#ip -6 route add 2001::/16 nexthop via fe80:2::3 dev swp1

store them as separate routes and pick the last one during lookup.

Newer kernels, after the support for equal cost multipath routes
was added http://patchwork.ozlabs.org/patch/188562/,
now accept multipath routes added individually using the
above 'ip -6 route' format OR
sent using RTA_MULTIPATH with the below command
#ip -6 route add 2001::/16 nexthop via fe80:2::2 dev swp1 nexthop via fe80:2::3 dev swp1

And the kernel now stores and treats them as equal cost multipath routes
during lookups.

However in all cases above, netlink notifications to ipv6 ECMP route adds and deletes
come separately for each next hop.

Example libnl notification in the above case with both old and new kernels:
	inet6 2001::/16 table main type unicast
                scope global priority 0x400 protocol boot
                nexthop via fe80:2::2 dev swp1

        inet6 2001::/16 table main type unicast
                scope global priority 0x400 protocol boot
                nexthop via fe80:2::3 dev swp1

Since they are separate route notifications for objects with same key,
libnl replaces the existing ones and hence only the last route update sticks.

This patch uses the oo_update feature to not replace but update an
existing route if its a ipv6 equal cost multipath route.
The object after an update looks like the below (similar to ipv4 ECMP routes):

        inet6 2001::/16 table main type unicast
                scope global priority 0x400 protocol boot
                nexthop via fe80:2::2 dev swp1
                nexthop via fe80:2::3 dev swp1

Signed-off-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: Nolan Leake <nolan@cumulusnetworks.com>
Reviewed-by: Wilson Kok <wkok@cumulusnetworks.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>