network: Use RTNH_COMPARE_MASK in route_can_update() (#36585)

Let's only compare flags that can be set by userspace and not all flags.

Fixes a bug introduced by 7027cdb (v256).
Fixes #36544

(cherry picked from commit 9fd25fd)

Author: DaanDeMeyer

src/network/networkd-route.c

@@ -1409,7 +1409,7 @@ bool route_can_update(const Route *existing, const Route *requesting) {
                         return false;
                 if (existing->type != requesting->type)
                         return false;
-                if (existing->flags != requesting->flags)
+                if ((existing->flags & ~RTNH_COMPARE_MASK) != (requesting->flags & ~RTNH_COMPARE_MASK))
                         return false;
                 if (!in6_addr_equal(&existing->prefsrc.in6, &requesting->prefsrc.in6))
                         return false;