nexthop: Rename artifacts related to legacy multipath nexthop groups

nexthop: Rename artifacts related to legacy multipath nexthop groups

After resilient next-hop groups have been added recently, there are two
types of multipath next-hop groups: the legacy "mpath", and the new
"resilient". Calling the legacy next-hop group type "mpath" is unfortunate,
because that describes the fact that a packet could be forwarded in one of
several paths, which is also true for the resilient next-hop groups.

Therefore, to make the naming clearer, rename various artifacts to reflect
the assumptions made. Therefore as of this patch:

- The flag for multipath groups is nh_grp_entry::is_multipath. This
  includes the legacy and resilient groups, as well as any future group
  types that behave as multipath groups.
  Functions that assume this have "mpath" in the name.

- The flag for legacy multipath groups is nh_grp_entry::hash_threshold.
  Functions that assume this have "hthr" in the name.

- The flag for resilient groups is nh_grp_entry::resilient.
  Functions that assume this have "res" in the name.

Besides the above, struct nh_grp_entry::mpath was renamed to ::hthr as
well.

UAPI artifacts were obviously left intact.

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