selftests/mm: pagemap_ioctl: Fix types mismatches shown by compiler options

Fix following warnings caught by compiler:

- There are several type mismatches among different variables.
- Remove unused variable warnings.

Link: https://lkml.kernel.org/r/20241209185624.2245158-3-usama.anjum@collabora.com


Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>