[retry] Use start_timer_fixed() instead of direct timeout manipulation

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv4.c

 
 		/* Set the reassembly timer */
 		timer_init ( &fragbuf->frag_timer, ipv4_frag_expired );
-		fragbuf->frag_timer.timeout = IP_FRAG_TIMEOUT;
-		start_timer ( &fragbuf->frag_timer );
+		start_timer_fixed ( &fragbuf->frag_timer, IP_FRAG_TIMEOUT );
 
 		/* Add the fragment buffer to the list of fragment buffers */
 		list_add ( &fragbuf->list, &frag_buffers );

src/net/tcp.c

 	 * timer to expire and cause the connection to be freed.
 	 */
 	if ( TCP_CLOSED_GRACEFULLY ( tcp->tcp_state ) ) {
-		tcp->timer.timeout = ( 2 * TCP_MSL );
-		start_timer ( &tcp->timer );
+		start_timer_fixed ( &tcp->timer, ( 2 * TCP_MSL ) );
 	}
 
 	return 0;