[netdevice] Add maximum packet length as a net device property

Currently this length is set at device allocation time, and is never
changed.

src/drivers/net/ipoib.c

  * IP over Infiniband
  */
 
-/** IPoIB MTU */
-#define IPOIB_MTU 2048
-
 /** Number of IPoIB data send work queue entries */
 #define IPOIB_DATA_NUM_SEND_WQES 2
 
 	int rc;
 
 	while ( qset->recv_fill < qset->recv_max_fill ) {
-		iobuf = alloc_iob ( IPOIB_MTU );
+		iobuf = alloc_iob ( IPOIB_PKT_LEN );
 		if ( ! iobuf )
 			break;
 		if ( ( rc = ib_post_recv ( ibdev, qset->qp, iobuf ) ) != 0 ) {

src/include/gpxe/ethernet.h

 
 #include <stdint.h>
 #include <gpxe/netdevice.h>
+#include <gpxe/if_ether.h>
 
 extern struct ll_protocol ethernet_protocol;
 
 	netdev = alloc_netdev ( priv_size );
 	if ( netdev ) {
 		netdev->ll_protocol = &ethernet_protocol;
+		netdev->max_pkt_len = ETH_FRAME_LEN;
 	}
 	return netdev;
 }

src/include/gpxe/ipoib.h

 
 #include <gpxe/infiniband.h>
 
+/** IPoIB packet length */
+#define IPOIB_PKT_LEN 2048
+
 /** IPoIB MAC address length */
 #define IPOIB_ALEN 20
 
 	netdev = alloc_netdev ( priv_size );
 	if ( netdev ) {
 		netdev->ll_protocol = &ipoib_protocol;
+		netdev->max_pkt_len = IPOIB_PKT_LEN;
 	}
 	return netdev;
 }

src/include/gpxe/netdevice.h

 	 * This is the bitwise-OR of zero or more NETDEV_XXX constants.
 	 */
 	unsigned int state;
+	/** Maximum packet length
+	 *
+	 * This length includes any link-layer headers.
+	 */
+	size_t max_pkt_len;
 	/** TX packet queue */
 	struct list_head tx_queue;
 	/** RX packet queue */