[efi] Provide guaranteed space in transmitted packets

eIPoIB requires space to expand a transmitted ARP packet.  This
guarantee is met by ensuring that a transmitted packet consists of at
least MAX_LL_HEADER_LEN bytes from the start of the I/O buffer up to
the end of the link-layer header, and at least IOB_ZLEN bytes
thereafter.

Adjust the I/O buffer allocation for SNP transmitted packets to ensure
that this guarantee is met.

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

src/interface/efi/efi_snp.c

 		container_of ( snp, struct efi_snp_device, snp );
 	struct ll_protocol *ll_protocol = snpdev->netdev->ll_protocol;
 	struct io_buffer *iobuf;
-	size_t ll_headroom;
+	size_t payload_len;
 	int rc;
 	EFI_STATUS efirc;
 
 	}
 
 	/* Allocate buffer */
-	ll_headroom = ( MAX_LL_HEADER_LEN - ll_header_len );
-	iobuf = alloc_iob ( ll_headroom +
-			    ( ( len > IOB_ZLEN ) ? len : IOB_ZLEN ) );
+	payload_len = ( len - ll_protocol->ll_header_len );
+	iobuf = alloc_iob ( MAX_LL_HEADER_LEN + ( ( payload_len > IOB_ZLEN ) ?
+						  payload_len : IOB_ZLEN ) );
 	if ( ! iobuf ) {
 		DBGC ( snpdev, "SNPDEV %p TX could not allocate %ld-byte "
 		       "buffer\n", snpdev, ( ( unsigned long ) len ) );
 		efirc = EFI_DEVICE_ERROR;
 		goto err_alloc_iob;
 	}
-	iob_reserve ( iobuf, ll_headroom );
+	iob_reserve ( iobuf, ( MAX_LL_HEADER_LEN -
+			       ll_protocol->ll_header_len ) );
 	memcpy ( iob_put ( iobuf, len ), data, len );
 
 	/* Create link-layer header, if specified */
 	if ( ll_header_len ) {
-		iob_pull ( iobuf, ll_header_len );
+		iob_pull ( iobuf, ll_protocol->ll_header_len );
 		if ( ( rc = ll_protocol->push ( snpdev->netdev,
 						iobuf, ll_dest, ll_src,
 						htons ( *net_proto ) )) != 0 ){