s390/mm: Allow large pages for KASAN shadow mapping

Commit c98d2eca ("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.

Add POPULATE_KASAN_MAP_SHADOW to the allowed list in large_allowed()
to restore large page mappings for KASAN shadows.

While large_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.

Fixes: c98d2eca ("s390/mm: Uncouple physical vs virtual address spaces")
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>