selftests/mm: fix va_high_addr_switch.sh failure on x86_64

The test will fail as below on x86_64 with cpu la57 support (will skip if
no la57 support). Note, the test requries nr_hugepages to be set first.

# running bash ./va_high_addr_switch.sh
# -------------------------------------
# mmap(addr_switch_hint - pagesize, pagesize): 0x7f55b60fa000 - OK
# mmap(addr_switch_hint - pagesize, (2 * pagesize)): 0x7f55b60f9000 - OK
# mmap(addr_switch_hint, pagesize): 0x800000000000 - OK
# mmap(addr_switch_hint, 2 * pagesize, MAP_FIXED): 0x800000000000 - OK
# mmap(NULL): 0x7f55b60f9000 - OK
# mmap(low_addr): 0x40000000 - OK
# mmap(high_addr): 0x1000000000000 - OK
# mmap(high_addr) again: 0xffff55b6136000 - OK
# mmap(high_addr, MAP_FIXED): 0x1000000000000 - OK
# mmap(-1): 0xffff55b6134000 - OK
# mmap(-1) again: 0xffff55b6132000 - OK
# mmap(addr_switch_hint - pagesize, pagesize): 0x7f55b60fa000 - OK
# mmap(addr_switch_hint - pagesize, 2 * pagesize): 0x7f55b60f9000 - OK
# mmap(addr_switch_hint - pagesize/2 , 2 * pagesize): 0x7f55b60f7000 - OK
# mmap(addr_switch_hint, pagesize): 0x800000000000 - OK
# mmap(addr_switch_hint, 2 * pagesize, MAP_FIXED): 0x800000000000 - OK
# mmap(NULL, MAP_HUGETLB): 0x7f55b5c00000 - OK
# mmap(low_addr, MAP_HUGETLB): 0x40000000 - OK
# mmap(high_addr, MAP_HUGETLB): 0x1000000000000 - OK
# mmap(high_addr, MAP_HUGETLB) again: 0xffff55b5e00000 - OK
# mmap(high_addr, MAP_FIXED | MAP_HUGETLB): 0x1000000000000 - OK
# mmap(-1, MAP_HUGETLB): 0x7f55b5c00000 - OK
# mmap(-1, MAP_HUGETLB) again: 0x7f55b5a00000 - OK
# mmap(addr_switch_hint - pagesize, 2*hugepagesize, MAP_HUGETLB): 0x800000000000 - FAILED
# mmap(addr_switch_hint , 2*hugepagesize, MAP_FIXED | MAP_HUGETLB): 0x800000000000 - OK
# [FAIL]

addr_switch_hint is defined as DFEFAULT_MAP_WINDOW in the failed test (for
x86_64, DFEFAULT_MAP_WINDOW is defined as (1UL<<47) - pagesize) in 64 bit.

Before commit cc92882ee218 ("mm: drop hugetlb_get_unmapped_area{_*}
functions"), for x86_64 hugetlb_get_unmapped_area() is handled in arch
code arch/x86/mm/hugetlbpage.c and addr is checked with
map_address_hint_valid() after align with 'addr &= huge_page_mask(h)'
which is a round down way, and it will fail the check because the addr is
within the DEFAULT_MAP_WINDOW but (addr + len) is above the
DFEFAULT_MAP_WINDOW. So it wil go through the
hugetlb_get_unmmaped_area_top_down() to find an area within the
DFEFAULT_MAP_WINDOW.

After commit cc92882ee218 ("mm: drop hugetlb_get_unmapped_area{_*}
functions"). The addr hint for hugetlb_get_unmmaped_area() will be
rounded up and aligned to hugepage size with ALIGN() for all arches. And
after the align, the addr will be above the default MAP_DEFAULT_WINDOW,
and the map_addresshint_valid() check will pass because both aligned addr
(addr0) and (addr + len) are above the DEFAULT_MAP_WINDOW, and the aligned
hint address (0x800000000000) is returned as an suitable gap is found
there, in arch_get_unmapped_area_topdown().

To still cover the case that addr is within the DEFAULT_MAP_WINDOW, and
addr + len is above the DFEFAULT_MAP_WINDOW, change to choose the last
hugepage aligned address within the DEFAULT_MAP_WINDOW as the hint addr,
and the addr + len (2 hugepages) will be one hugepage above the
DEFAULT_MAP_WINDOW. An aligned address won't be affected by the page
round up or round down from kernel, so it's determistic.

Link: https://lkml.kernel.org/r/20250912013711.3002969-4-chuhu@redhat.com
Fixes: cc92882ee218 ("mm: drop hugetlb_get_unmapped_area{_*} functions")
Signed-off-by: Chunyu Hu <chuhu@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>