Rename pkb_available() to pkb_tailroom() for consistency with Linux's

skb_tailroom().  Add pkb_headroom().

src/arch/i386/drivers/net/undinet.c

 				/* Fragment will be dropped */
 				goto done;
 			}
-			if ( frag_len > pkb_available ( pkb ) ) {
+			if ( frag_len > pkb_tailroom ( pkb ) ) {
 				DBGC ( undinic, "UNDINIC %p fragment too "
 				       "large\n", undinic );
-				frag_len = pkb_available ( pkb );
+				frag_len = pkb_tailroom ( pkb );
 			}
 			copy_from_real ( pkb_put ( pkb, frag_len ),
 					 undi_isr.Frame.segment,

src/include/gpxe/pkbuff.h

 }
 
 /**
- * Calculate available space in a packet buffer
+ * Calculate available space at start of a packet buffer
  *
  * @v pkb	Packet buffer
- * @ret len	Length of data available in buffer
+ * @ret len	Length of data available at start of buffer
  */
-static inline size_t pkb_available ( struct pk_buff *pkb ) {
+static inline size_t pkb_headroom ( struct pk_buff *pkb ) {
+	return ( pkb->data - pkb->head );
+}
+
+/**
+ * Calculate available space at end of a packet buffer
+ *
+ * @v pkb	Packet buffer
+ * @ret len	Length of data available at end of buffer
+ */
+static inline size_t pkb_tailroom ( struct pk_buff *pkb ) {
 	return ( pkb->end - pkb->tail );
 }
 

src/net/tcp.c

 	if ( TCP_CAN_SEND_DATA ( conn->tcp_state ) &&
 	     app && app->tcp_op->senddata ) {
 		conn->tx_pkb = pkb;
-		app->tcp_op->senddata ( app, pkb->data, pkb_available ( pkb ));
+		app->tcp_op->senddata ( app, pkb->data, pkb_tailroom ( pkb ) );
 		conn->tx_pkb = NULL;
 	}
 
 	}
 
 	/* Truncate length to fit packet buffer */
-	if ( len > pkb_available ( pkb ) )
-		len = pkb_available ( pkb );
+	if ( len > pkb_tailroom ( pkb ) )
+		len = pkb_tailroom ( pkb );
 
 	/* Copy payload */
 	memmove ( pkb_put ( pkb, len ), data, len );

src/net/udp.c

 	}
 	pkb_reserve ( conn->tx_pkb, UDP_MAX_HLEN );
 	rc = conn->udp_op->senddata ( conn, conn->tx_pkb->data, 
-				      pkb_available ( conn->tx_pkb ) );
+				      pkb_tailroom ( conn->tx_pkb ) );
 	if ( conn->tx_pkb )
 		free_pkb ( conn->tx_pkb );
 	return rc;
 	conn->tx_pkb = NULL;
 
 	/* Avoid overflowing TX buffer */
-	if ( len > pkb_available ( pkb ) )
-		len = pkb_available ( pkb );
+	if ( len > pkb_tailroom ( pkb ) )
+		len = pkb_tailroom ( pkb );
 
 	/* Copy payload */
 	memmove ( pkb_put ( pkb, len ), data, len );