netlink: always zero provided header buffers.

Reported by:	KMSAN
MFC after:	3 days

sys/netlink/netlink_message_writer.h

@@ -225,6 +225,7 @@ nlmsg_reserve_data_raw(struct nl_writer *nw, size_t sz)
 
         void *data_ptr = &nw->data[nw->offset];
         nw->offset += sz;
+	bzero(data_ptr, sz);
 
         return (data_ptr);
 }

sys/netlink/route/neigh.c

@@ -156,12 +156,9 @@ dump_lle_locked(struct llentry *lle, void *arg)
 
 	ndm = nlmsg_reserve_object(nw, struct ndmsg);
 	ndm->ndm_family = wa->family;
-	ndm->ndm_pad1 = 0;
-	ndm->ndm_pad2 = 0;
 	ndm->ndm_ifindex = wa->ifp->if_index;
 	ndm->ndm_state = lle_state_to_nl_state(wa->family, lle);
 	ndm->ndm_flags = lle_flags_to_nl_flags(lle);
-	ndm->ndm_type = 0;
 
 	switch (wa->family) {
 #ifdef INET