selftests/mm: ksm_tests should only MADV_HUGEPAGE valid memory

ksm_tests was previously mmapping a region of memory, aligning the
returned pointer to a PMD boundary, then setting MADV_HUGEPAGE, but was
setting it past the end of the mmapped area due to not taking the pointer
alignment into consideration.  Fix this behaviour.

Up until commit efa7df3 ("mm: align larger anonymous mappings on THP
boundaries"), this buggy behavior was (usually) masked because the
alignment difference was always less than PMD-size.  But since the
mentioned commit, `ksm_tests -H -s 100` started failing.

Link: https://lkml.kernel.org/r/20240122120554.3108022-1-ryan.roberts@arm.com
Fixes: 3252548 ("selftests: vm: add KSM huge pages merging time test")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ryanhrob

tools/testing/selftests/mm/ksm_tests.c

@@ -566,7 +566,7 @@ static int ksm_merge_hugepages_time(int merge_type, int mapping, int prot,
 	if (map_ptr_orig == MAP_FAILED)
 		err(2, "initial mmap");
 
-	if (madvise(map_ptr, len + HPAGE_SIZE, MADV_HUGEPAGE))
+	if (madvise(map_ptr, len, MADV_HUGEPAGE))
 		err(2, "MADV_HUGEPAGE");
 
 	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);