nexthop: Add netlink defines and enumerators for resilient NH groups

- RTM_NEWNEXTHOP et.al. that handle resilient groups will have a new nested
  attribute, NHA_RES_GROUP, whose elements are attributes NHA_RES_GROUP_*.

- RTM_NEWNEXTHOPBUCKET et.al. is a suite of new messages that will
  currently serve only for dumping of individual buckets of resilient next
  hop groups. For nexthop group buckets, these messages will carry a nested
  attribute NHA_RES_BUCKET, whose elements are attributes NHA_RES_BUCKET_*.

  There are several reasons why a new suite of messages is created for
  nexthop buckets instead of overloading the information on the existing
  RTM_{NEW,DEL,GET}NEXTHOP messages.

  First, a nexthop group can contain a large number of nexthop buckets (4k
  is not unheard of). This imposes limits on the amount of information that
  can be encoded for each nexthop bucket given a netlink message is limited
  to 64k bytes.

  Second, while RTM_NEWNEXTHOPBUCKET is only used for notifications at
  this point, in the future it can be extended to provide user space with
  control over nexthop buckets configuration.

- The new group type is NEXTHOP_GRP_TYPE_RES. Note that nexthop code is
  adjusted to bounce groups with that type for now.

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>