core: set routing rule protocol properly

When creating NMPlatformRoutingRule from NMIPRouteRule object, the
protocol is being set to RTPROT_UNSPEC. According to linux kernel
documentation FRA_PROTOCOL indicates the originator of the rule.

In this case the route rule is coming from a connection and therefore
the originator of the rule is the user. The correct value is
RTPROT_STATIC which means the rule is installed by the administrator.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1457

Fixes: 3f9347745b ('core: add handling of IP routing rules to NMDevice')
(cherry picked from commit 82009e21d2)

src/core/NetworkManagerUtils.c

@@ -978,6 +978,7 @@ nm_ip_routing_rule_to_platform(const NMIPRoutingRule *rule, NMPlatformRoutingRul
                 .start = uid_range_start,
                 .end   = uid_range_end,
             },
+        .protocol = RTPROT_STATIC,
     };
 
     nm_ip_routing_rule_get_xifname_bin(rule, TRUE, out_pl->iifname);