nexthop: Add data structures for resilient group notifications

Add data structures that will be used for in-kernel notifications about
addition / deletion of a resilient nexthop group and about changes to a
hash bucket within a resilient group.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>