[xfer] Use xfer_alloc_iob() for transmit I/O buffers on stream sockets

Datagram sockets such as UDP, ICMP, and fibre channel tend to provide
a custom xfer_alloc_iob() handler to ensure that transmit I/O buffers
contain sufficient headroom to accommodate any required protocol
headers.

Stream sockets such as TCP and TLS do not typically provide a custom
xfer_alloc_iob() handler at present.  The default handler simply calls
alloc_iob(), and so stream socket consumers can therefore get away
with using alloc_iob() rather than xfer_alloc_iob().

Fix the HTTP and ONC RPC protocols to use xfer_alloc_iob() where
relevant, in order to operate correctly if the underlying stream
socket chooses to provide a custom xfer_alloc_iob() handler.

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

src/net/tcp/httpcore.c

@@ -1090,7 +1090,8 @@ static int http_tx_request ( struct http_transaction *http ) {
 	}
 
 	/* Allocate I/O buffer */
-	iobuf = alloc_iob ( len + 1 /* NUL */ + http->request.content.len );
+	iobuf = xfer_alloc_iob ( &http->conn, ( len + 1 /* NUL */ +
+						http->request.content.len ) );
 	if ( ! iobuf ) {
 		rc = -ENOMEM;
 		goto err_alloc;

src/net/tcp/oncrpc.c

@@ -150,7 +150,7 @@ int oncrpc_call ( struct interface *intf, struct oncrpc_session *session,
 	frame_size  = oncrpc_compute_size ( header );
 	frame_size += oncrpc_compute_size ( fields );
 
-	io_buf = alloc_iob ( frame_size );
+	io_buf = xfer_alloc_iob ( intf, frame_size );
 	if ( ! io_buf )
 		return -ENOBUFS;