nexthop: Implement notifiers for resilient nexthop groups

Implement the following notifications towards drivers:

- NEXTHOP_EVENT_REPLACE, when a resilient nexthop group is created.

- NEXTHOP_EVENT_BUCKET_REPLACE any time there is a change in assignment of
  next hops to hash table buckets. That includes replacements, deletions,
  and delayed upkeep cycles. Some bucket notifications can be vetoed by the
  driver, to make it possible to propagate bucket busy-ness flags from the
  HW back to the algorithm. Some are however forced, e.g. if a next hop is
  deleted, all buckets that use this next hop simply must be migrated,
  whether the HW wishes so or not.

- NEXTHOP_EVENT_RES_TABLE_PRE_REPLACE, before a resilient nexthop group is
  replaced. Usually the driver will get the bucket notifications as well,
  and could veto those. But in some cases, a bucket may not be migrated
  immediately, but during delayed upkeep, and that is too late to roll the
  transaction back. This notification allows the driver to take a look and
  veto the new proposed group up front, before anything is committed.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>