selftests/mm: fix faulting-in code in pagemap_ioctl test

selftests/mm: fix faulting-in code in pagemap_ioctl test

One of the pagemap_ioctl tests attempts to fault in pages by memcpy()'ing
them to an unused buffer.  This probably worked originally, but since
commit 46036188ea1f ("selftests/mm: build with -O2") the compiler is free
to optimise away that unused buffer and the memcpy() with it.  As a result
there might not be any resident page in the mapping and the test may fail.

We don't need to copy all that memory anyway.  Just fault in every page.

While at it also make sure to compute the number of pages once using
simple integer arithmetic instead of ceilf() and implicit conversions.

Link: https://lkml.kernel.org/r/20260122170224.4056513-8-kevin.brodsky@arm.com
Fixes: 46036188ea1f ("selftests/mm: build with -O2")
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: wang lian <lianux.mm@gmail.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>