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: Alison Schofield <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/vm/gup_benchmark.c

···
 	__u64 size;
 	__u32 nr_pages_per_call;
 	__u32 flags;
+	__u64 expansion[10];	/* For future use */
 };
 
 int main(int argc, char **argv)