cxl: Use fixed width predefined types in data structure.

This patch fixes a regression introduced by commit b810253b ("cxl:
Add mechanism for delivering AFU driver specific events").

It changes the type u8 to __u8 in the uapi header cxl.h, because the
former is a kernel internal type, and may not be defined in userland
build environments, in particular when cross-compiling libcxl on x86_64
linux machines (RHEL6.7 and Ubuntu 16.04).

This patch also changes the size of the field data_size, and makes it
constant, to support 32-bit userland applications running on big-endian
ppc64 kernels transparently.

mpe: This is an ABI change, however the ABI was only added during the
4.8 merge window so has never been part of a released kernel - therefore
we give ourselves permission to change it.

Fixes: b810253b ("cxl: Add mechanism for delivering AFU driver specific events")
Signed-off-by: Philippe Bergheaud <felix@linux.vnet.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

include/uapi/misc/cxl.h

@@ -136,8 +136,8 @@ struct cxl_event_afu_driver_reserved {
 	 *
 	 * Of course the contents will be ABI, but that's up the AFU driver.
 	 */
-	size_t data_size;
-	u8 data[];
+	__u32 data_size;
+	__u8 data[];
 };
 
 struct cxl_event {