selftest/mm: fix pointer comparison in mremap_test

selftest/mm: fix pointer comparison in mremap_test

Pointer arthemitic with 'void * addr' and 'ulong dest_alignment' triggers
following warning:

mremap_test.c:1035:31: warning: pointer comparison always evaluates to
false [-Wtautological-compare]
 1035 |                 if (addr + c.dest_alignment < addr) {
      |                                             ^

this warning is raised from clang version 20.1.8 (Fedora 20.1.8-4.fc42).

use 'void *tmp_addr' to do the pointer arthemitic.

Link: https://lkml.kernel.org/r/20251108161829.25105-1-ankitkhushwaha.linux@gmail.com
Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>