[iobuf] Add iob_disown() and use it where it simplifies code

There are many functions that take ownership of the I/O buffer they
are passed as a parameter.  The caller should not retain a pointer to
the I/O buffer.  Use iob_disown() to automatically nullify the
caller's pointer, e.g.:

    xfer_deliver_iob ( xfer, iob_disown ( iobuf ) );

This will ensure that iobuf is set to NULL for any code after the call
to xfer_deliver_iob().

iob_disown() is currently used only in places where it simplifies the
code, by avoiding an extra line explicitly setting the I/O buffer
pointer to NULL.  It should ideally be used with each call to any
function that takes ownership of an I/O buffer.  (The SSA
optimisations will ensure that use of iob_disown() gets optimised away
in cases where the caller makes no further use of the I/O buffer
pointer anyway.)

If gcc ever introduces an __attribute__((free)), indicating that use
of a function argument after a function call should generate a
warning, then we should use this to identify all applicable function
call sites, and add iob_disown() as necessary.

src/interface/efi/efi_snp.c

@@ -602,10 +602,9 @@ efi_snp_transmit ( EFI_SIMPLE_NETWORK_PROTOCOL *snp,
 	}
 
 	/* Transmit packet */
-	if ( ( rc = netdev_tx ( snpdev->netdev, iobuf ) ) != 0 ) {
+	if ( ( rc = netdev_tx ( snpdev->netdev, iob_disown ( iobuf ) ) ) != 0){
 		DBGC ( snpdev, "SNPDEV %p TX could not transmit: %s\n",
 		       snpdev, strerror ( rc ) );
-		iobuf = NULL;
 		efirc = RC_TO_EFIRC ( rc );
 		goto err_tx;
 	}