Fix a couple of broken assertions, and align the buffer correctly.

src/include/gpxe/pkbuff.h

  * This structure is used to represent a network packet within gPXE.
  */
 struct pk_buff {
-	/** Head of the buffer */
+	/** Start of the buffer */
 	void *head;
 	/** Start of data */
 	void *data;
  */
 static inline void * pkb_pull ( struct pk_buff *pkb, size_t len ) {
 	pkb->data += len;
-	assert ( pkb->data >= pkb->tail );
+	assert ( pkb->data <= pkb->tail );
 	return pkb->data;
 }
 

src/net/pkbuff.c

 	void *data;
 
 	/* Align buffer length */
-	len = ( len + __alignof__ ( *pkb ) - 1 ) & ~ __alignof__ ( *pkb );
+	len = ( len + __alignof__( *pkb ) - 1 ) & ~( __alignof__( *pkb ) - 1 );
 	
 	/* Allocate memory for buffer plus descriptor */
 	data = malloc_dma ( len + sizeof ( *pkb ), PKBUFF_ALIGN );
  */
 void free_pkb ( struct pk_buff *pkb ) {
 	if ( pkb ) {
+		assert ( pkb->head <= pkb->data );
+		assert ( pkb->data <= pkb->tail );
+		assert ( pkb->tail <= pkb->end );
 		free_dma ( pkb->head,
 			   ( pkb->end - pkb->head ) + sizeof ( *pkb ) );
 	}