selftests/mm: hugetlb_madv_vs_map: avoid test skipping by querying hugepage size at runtime

Currently, the size used in mmap() is statically defined, leading to
skipping of the test on a hugepage size other than 2 MB, since munmap()
won't free the hugepage for a size greater than 2 MB.  Hence, query the
size at runtime.

Also, there is no reason why a hugepage allocation should fail, since we
are using a simple mmap() using MAP_HUGETLB; hence, instead of skipping
the test, make it fail.

Link: https://lkml.kernel.org/r/20240509095447.3791573-1-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>