[dhcp] Remove redundant length fields in struct dhcp_packet

The max_len field is never used, and the len field is used only by
dhcp_tx().  Remove these two fields, and perform the necessary trivial
calculation in dhcp_tx() instead.

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

src/include/ipxe/dhcppkt.h

 	struct refcnt refcnt;
 	/** The DHCP packet contents */
 	struct dhcphdr *dhcphdr;
-	/** Maximum length of the DHCP packet buffer */
-	size_t max_len;
-	/** Used length of the DHCP packet buffer */
-	size_t len;
 	/** DHCP options */
 	struct dhcp_options options;
 	/** Settings interface */
 	ref_put ( &dhcppkt->refcnt );
 }
 
+/**
+ * Get used length of DHCP packet
+ *
+ * @v dhcppkt		DHCP packet
+ * @ret len		Used length
+ */
+static inline int dhcppkt_len ( struct dhcp_packet *dhcppkt ) {
+	return ( offsetof ( struct dhcphdr, options ) + dhcppkt->options.len );
+}
+
 extern int dhcppkt_store ( struct dhcp_packet *dhcppkt, unsigned int tag,
 			   const void *data, size_t len );
 extern int dhcppkt_fetch ( struct dhcp_packet *dhcppkt, unsigned int tag,

src/net/dhcppkt.c

 		    const void *data, size_t len ) {
 	struct dhcp_packet_field *field;
 	void *field_data;
-	int rc;
 
 	/* If this is a special field, fill it in */
 	if ( ( field = find_dhcp_packet_field ( tag ) ) != NULL ) {
 	}
 
 	/* Otherwise, use the generic options block */
-	rc = dhcpopt_store ( &dhcppkt->options, tag, data, len );
-
-	/* Update our used-length field */
-	dhcppkt->len = ( offsetof ( struct dhcphdr, options ) +
-			 dhcppkt->options.len );
-
-	return rc;
+	return dhcpopt_store ( &dhcppkt->options, tag, data, len );
 }
 
 /**
 		    size_t len ) {
 	ref_init ( &dhcppkt->refcnt, NULL );
 	dhcppkt->dhcphdr = data;
-	dhcppkt->max_len = len;
 	dhcpopt_init ( &dhcppkt->options, &dhcppkt->dhcphdr->options,
 		       ( len - offsetof ( struct dhcphdr, options ) ) );
-	dhcppkt->len = ( offsetof ( struct dhcphdr, options ) +
-			 dhcppkt->options.len );
 	settings_init ( &dhcppkt->settings,
 			&dhcppkt_settings_operations, &dhcppkt->refcnt, 0 );
 }

src/net/udp/dhcp.c

 	}
 
 	/* Transmit the packet */
-	iob_put ( iobuf, dhcppkt.len );
+	iob_put ( iobuf, dhcppkt_len ( &dhcppkt ) );
 	if ( ( rc = xfer_deliver ( &dhcp->xfer, iob_disown ( iobuf ),
 				   &meta ) ) != 0 ) {
 		DBGC ( dhcp, "DHCP %p could not transmit UDP packet: %s\n",