bsnmpd: Only refresh devtree if devd event is a new or removed device.

It makes sense to refresh the tree only when a device is inserted or
removed, otherwise bsnmpd wastes lot of CPU.

PR:		209368
MFC after:	1 week

usr.sbin/bsnmpd/modules/snmp_hostres/hostres_device_tbl.c

@@ -479,7 +479,9 @@ devd_socket_callback(int fd, void *arg __unused)
 	} else {
 		if (read_len == sizeof(buf))
 			goto again;
-		refresh_device_tbl(1);
+		/* Only refresh device table on a device add or remove event. */
+		if (buf[0] == '+' || buf[0] == '-')
+			refresh_device_tbl(1);
 	}
 }