selftests/mm: fix incorrect buffer->mirror size in hmm2 double_map test

selftests/mm: fix incorrect buffer->mirror size in hmm2 double_map test

The hmm2 double_map test was failing due to an incorrect buffer->mirror
size.  The buffer->mirror size was 6, while buffer->ptr size was 6 *
PAGE_SIZE.  The test failed because the kernel's copy_to_user function was
attempting to copy a 6 * PAGE_SIZE buffer to buffer->mirror.  Since the
size of buffer->mirror was incorrect, copy_to_user failed.

This patch corrects the buffer->mirror size to 6 * PAGE_SIZE.

Test Result without this patch
==============================
 #  RUN           hmm2.hmm2_device_private.double_map ...
 # hmm-tests.c:1680:double_map:Expected ret (-14) == 0 (0)
 # double_map: Test terminated by assertion
 #          FAIL  hmm2.hmm2_device_private.double_map
 not ok 53 hmm2.hmm2_device_private.double_map

Test Result with this patch
===========================
 #  RUN           hmm2.hmm2_device_private.double_map ...
 #            OK  hmm2.hmm2_device_private.double_map
 ok 53 hmm2.hmm2_device_private.double_map

Link: https://lkml.kernel.org/r/20240927050752.51066-1-donettom@linux.ibm.com
Fixes: fee9f6d1b8df ("mm/hmm/test: add selftests for HMM")
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>