netlink: restore the ability to delete PINNED routes

route(8) had such ability before migration to netlink. To be able
to delete PINNED routes we pass RTM_F_FORCE to rib_del_route_px()
when userland uses RTF_PINNED flag with RTM_DELETE command.
This is hackish way for route(8) tool, that third-party software
usually doesn't use.

PR:		279988
Tested by:	franco at opnsense org
Discussed with:	glebius
MFC after:	10 days
Differential Revision:	https://reviews.freebsd.org/D46301

sys/netlink/route/rt.c

@@ -1012,7 +1012,8 @@ rtnl_handle_delroute(struct nlmsghdr *hdr, struct nlpcb *nlp,
 	}
 
 	error = rib_del_route_px(attrs.rta_table, attrs.rta_dst,
-	    attrs.rtm_dst_len, path_match_func, &attrs, 0, &rc);
+	    attrs.rtm_dst_len, path_match_func, &attrs,
+	    (attrs.rta_rtflags & RTF_PINNED) ? RTM_F_FORCE : 0, &rc);
 	if (error == 0)
 		report_operation(attrs.rta_table, &rc, nlp, hdr);
 	return (error);