selftests/vm/gup_benchmark.c: match gup struct to kernel

An expansion field was added to the kernel copy of this structure for
future use. See mm/gup_benchmark.c.

Add the same expansion field here, so that the IOCTL command decodes
correctly. Otherwise, it fails with EINVAL.
Signed-off-by: NAlison Schofield <alison.schofield@intel.com>
Acked-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: NShuah Khan <shuah@kernel.org>

tools/testing/selftests/vm/gup_benchmark.c

@@ -25,6 +25,7 @@ struct gup_benchmark {
 	__u64 size;
 	__u32 nr_pages_per_call;
 	__u32 flags;
+	__u64 expansion[10];	/* For future use */
 };
 
 int main(int argc, char **argv)