[efi] Match behaviour of SnpDxe for truncated received packets

The UEFI specification does not state whether or not a return value of
EFI_BUFFER_TOO_SMALL from the SNP Receive() method should follow the
usual EFI API behaviour of allowing the caller to retry the request
with an increased buffer size.

Examination of the SnpDxe driver in EDK2 suggests that Receive() will
just return the truncated packet (complete with any requested
link-layer header fields), so match this behaviour.

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

src/interface/efi/efi_snp.c

 	const void *iob_ll_src;
 	uint16_t iob_net_proto;
 	unsigned int iob_flags;
-	size_t max_len;
+	size_t copy_len;
 	int rc;
 
 	DBGC2 ( snpdev, "SNPDEV %p RECEIVE %p(+%lx)", snpdev, data,
 	}
 	DBGC2 ( snpdev, "+%zx\n", iob_len ( iobuf ) );
 
-	/* Check buffer length */
-	max_len = *len;
-	*len = iob_len ( iobuf );
-	if ( *len > max_len ) {
-		rc = -ERANGE;
-		goto out_too_long;
-	}
-
 	/* Dequeue packet */
 	list_del ( &iobuf->list );
 
-	/* Return packet to caller */
-	memcpy ( data, iobuf->data, iob_len ( iobuf ) );
+	/* Return packet to caller, truncating to buffer length */
+	copy_len = iob_len ( iobuf );
+	if ( copy_len > *len )
+		copy_len = *len;
+	memcpy ( data, iobuf->data, copy_len );
+	*len = iob_len ( iobuf );
 
 	/* Attempt to decode link-layer header */
 	if ( ( rc = ll_protocol->pull ( snpdev->netdev, iobuf, &iob_ll_dest,
 	if ( net_proto )
 		*net_proto = ntohs ( iob_net_proto );
 
-	rc = 0;
+	/* Check buffer length */
+	rc = ( ( copy_len == *len ) ? 0 : -ERANGE );
 
  out_bad_ll_header:
 	free_iob ( iobuf );
- out_too_long:
  out_no_packet:
 	return EFIRC ( rc );
 }