mm/selftests: fix pagemap_ioctl memory map test

__FILE__ is not guaranteed to exist in current dir.  Replace that with
argv[0] for memory map test.

Link: https://lkml.kernel.org/r/20231116201547.536857-4-peterx@redhat.com
Fixes: 46fd75d4

 ("selftests: mm: add pagemap ioctl tests")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>