selftests/mm: mremap_test: Remove unused variable and type mismatches

Remove unused variable and fix type mismatches.

Link: https://lkml.kernel.org/r/20241209185624.2245158-5-usama.anjum@collabora.com


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