selftests/mm: pagemap_ioctl: use the correct page size for transact_test()

selftests/mm: pagemap_ioctl: use the correct page size for transact_test()

There are several places in transact_test() where we use the hardcoded
0x1000 (4k) as page size, which is not always correct for architectures
supporting multiple page sizes.

Switch to use the correct page size.  Otherwise ./ksft_pagemap.sh on a
16k-page-size arm64 box fails with

 $ ./ksft_pagemap.sh
 [...]
 # ok 96 mprotect_tests Both pages written after remap and mprotect
 # ok 97 mprotect_tests Clear and make the pages written
 # Bail out! ioctl failed
 # # Planned tests != run tests (117 != 97)
 # # Totals: pass:97 fail:0 xfail:0 xpass:0 skip:0 error:0
 # [FAIL]
 not ok 1 pagemap_ioctl # exit=1
 # SUMMARY: PASS=0 SKIP=0 FAIL=1
 1..1

Link: https://lore.kernel.org/20260628101118.35861-1-zenghui.yu@linux.dev
Fixes: 46fd75d4a3c9 ("selftests: mm: add pagemap ioctl tests")
Signed-off-by: Zenghui Yu <zenghui.yu@linux.dev>
Cc: Muhammad Usama Anjum <usama.anjum@arm.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
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@kernel.org>
Cc: Zenghui Yu <zenghui.yu@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>