[xferbuf] Record maximum required size

Record the maximum size required when writing into a data transfer
buffer.  This allows the maximum size to be determined even if
allocation fails (e.g. due to a fixed-size buffer or an out-of-memory
condition).

In the case of a fixed-size buffer (which may already be larger than
required), this allows the caller to determine the actual size used
for written data.

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

src/include/ipxe/xferbuf.h

@@ -21,6 +21,8 @@ struct xfer_buffer {
 	void *data;
 	/** Size of data */
 	size_t len;
+	/** Maximum required size of data */
+	size_t max;
 	/** Current offset within data */
 	size_t pos;
 	/** Data transfer buffer operations */