lib/test_hmm: reduce stack usage

lib/test_hmm: reduce stack usage

The various test ioctl handlers use arrays of 64 integers that add up to
1KiB of stack data, which in turn leads to exceeding the warning limit in
some configurations:

lib/test_hmm.c:935:12: error: stack frame size (1408) exceeds limit (1280)
in 'dmirror_migrate_to_device' [-Werror,-Wframe-larger-than]

Use half the size for these arrays, in order to stay under the warning
limits.  The code can already deal with arbitrary lengths, but this may be
a little less efficient.

Link: https://lkml.kernel.org/r/20250610092159.2639515-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jeff Johnson <jeff.johnson@oss.qualcomm.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>