selftests/mm: use calloc instead of malloc in pagemap_ioctl.c

selftests/mm: use calloc instead of malloc in pagemap_ioctl.c

As per Documentation/process/deprecated.rst, dynamic size calculations
should not be performed in memory allocator arguments due to possible
overflows.

Replace malloc with calloc to avoid open-ended arithmetic and prevent
possible overflows.

Link: https://lkml.kernel.org/r/20250825170643.63174-1-viswanathiyyappan@gmail.com
Signed-off-by: I Viswanath <viswanathiyyappan@gmail.com>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed by: Donet Tom <donettom@linux.ibm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
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>