ipv6: Export sernum update function

We are going to allow dead routes to stay in the FIB tree (e.g., when
they are part of a multipath route, directly connected route with no
carrier) and revive them when their nexthop device gains carrier or when
it is put administratively up.

This is equivalent to the addition of the route to the FIB tree and we
should therefore take care of updating the sernum of all the parent
nodes of the node where the route is stored. Otherwise, we risk sockets
caching and using sub-optimal dst entries.

Export the function that performs the above, so that it could be invoked
from fib6_ifup() later on.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>