nexthop: Allow setting "offload" and "trap" indication of nexthop buckets

Add a function that can be called by device drivers to set "offload" or
"trap" indication on nexthop buckets following nexthop notifications and
other changes such as a neighbour becoming invalid.

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>