selftests/vm: cow: fix compile warning on 32bit

The compiler complains about the conversion of a pointer to an int of
different width.

Link: https://lkml.kernel.org/r/20221205193716.276024-4-david@redhat.com


Fixes: 6f1405ef ("selftests/vm: anon_cow: add R/O longterm tests via gup_test")
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>