ip6mr: Add API for default_rule fib

Add the ability to discern whether a given FIB rule notification relates
to the default rule inserted when registering ip6mr or a different one.

Would later be used by drivers wishing to offload ipv6 multicast routes
but unable to offload rules other than the default one.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>