mm: Convert zero page to large folios on write

A read fault causes the zero page to be mapped read-only. A subsequent
write fault causes the zero page to be replaced with a zero-filled
private anonymous page. Change the write fault behaviour to replace the
zero page with a large anonymous folio, allocated using the same policy
as if the write fault had happened without the previous read fault.

Experimentation shows that reading multiple contiguous pages is
extremely rare without interleved writes, so we don't bother to map a
large zero page. We just use the small zero page as a marker and expand
the allocation at the write fault.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>