s390/mm: Allow large pages for KASAN shadow mapping

s390/mm: Allow large pages for KASAN shadow mapping

Commit c98d2ecae08f ("s390/mm: Uncouple physical vs virtual address
spaces") introduced a large_allowed() helper that restricts which mapping
modes can use large pages. This change unintentionally prevented KASAN
shadow mappings from using large pages, despite there being no reason
to avoid them. In fact, large pages are preferred for performance.

Since commit d8073dc6bc04 ("s390/mm: Allow large pages only for aligned
physical addresses"), both can_large_pud() and can_large_pmd() call _pa()
to check if large page physical addresses are aligned. However, _pa()
has a side effect: it allocates memory in POPULATE_KASAN_MAP_SHADOW
mode.

Rename large_allowed() to large_page_mapping_allowed() and add
POPULATE_KASAN_MAP_SHADOW to the allowed list to restore large page
mappings for KASAN shadows.

While large_page_mapping_allowed() isn't strictly necessary with current
mapping modes since disallowed modes either don't map anything or fail
alignment and size checks, keep it for clarity.

Rename _pa() to resolve_pa_may_alloc() for clarity and to emphasize
existing side effect. Rework can_large_pud()/can_large_pmd() to take
the side effect into consideration and actually return physical address
instead of just checking conditions.

Fixes: c98d2ecae08f ("s390/mm: Uncouple physical vs virtual address spaces")
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>