nl_l3: fix nh_stub::operator<() strict weak ordering

According to [1], std::set requires a Compare function that has strict
weak ordering [2], but the current implementation violates the second
requirement:

* If comp(a, b) == true then comp(b, a) == false.

E.g. given the nh_stubs a = <169.254.0.1,62> and b = <172.16.111.1,4>:

* comp(a, b) is true, as 172.16.111.1 isn't < 169.254.0.1, but 4 < 62
* comp(b, a) is true, as 169.254.0.1 < 172.16.111.1.

This breaks e.g. lookups in sets, causing element not being deleted or
being found.

Fix this by comparing the ifindex only if the addresses are equal, not
only if the other one's is larger.

[1] https://en.cppreference.com/w/cpp/container/set
[2] https://en.cppreference.com/w/cpp/named_req/Compare

Fixes: 0fdba0d ("nl_l3: only route l3 neighs if we have a route for them")
Signed-off-by: Jonas Gorski <jonas.gorski@bisdn.de>

src/netlink/nl_l3_interfaces.h

@@ -52,8 +52,12 @@ struct nh_stub {
   }
 
   bool operator<(const nh_stub &other) const {
-    if (nl_addr_cmp(nh, other.nh) < 0)
+    int cmp = nl_addr_cmp(nh, other.nh);
+
+    if (cmp < 0)
       return true;
+    if (cmp > 0)
+      return false;
 
     return ifindex < other.ifindex;
   }