zebra: Log how we got to this failure.

When a filter function fails to work correctly, we get an
error message that something has gone wrong.  Unfortunately
we may not have any clues as to where the decode failure
happened.  Add a backtrace to give us a clue.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

zebra/kernel_netlink.c

@@ -772,6 +772,7 @@ int netlink_parse_info(int (*filter)(struct nlmsghdr *, ns_id_t, int),
 			error = (*filter)(h, zns->ns_id, startup);
 			if (error < 0) {
 				zlog_err("%s filter function error", nl->name);
+				zlog_backtrace(LOG_ERR);
 				ret = error;
 			}
 		}