nexthop: Assert the invariant that a NH group is of only one type

Most of the code that deals with nexthop groups relies on the fact that the
group is of exactly one well-known type. Currently there is only one type,
"mpath", but as more next-hop group types come, it becomes desirable to
have a central place where the setting is validated. Introduce such place
into nexthop_create_group(), such that the check is done before the code
that relies on that invariant is invoked.
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: Jakub Kicinski <kuba@kernel.org>

net/ipv4/nexthop.c

@@ -1466,10 +1466,13 @@ static struct nexthop *nexthop_create_group(struct net *net,
 		nhg->nh_entries[i].nh_parent = nh;
 	}
 
-	if (cfg->nh_grp_type == NEXTHOP_GRP_TYPE_MPATH) {
+	if (cfg->nh_grp_type == NEXTHOP_GRP_TYPE_MPATH)
 		nhg->mpath = 1;
+
+	WARN_ON_ONCE(nhg->mpath != 1);
+
+	if (nhg->mpath)
 		nh_group_rebalance(nhg);
-	}
 
 	if (cfg->nh_fdb)
 		nhg->fdb_nh = 1;