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 <alison.schofield@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>