mm: Copy large folios for anonymous memory

When taking a write fault on an anonymous page, if we are unable to
reuse the folio (due to it being mapped by others), do CoW for the
entire folio instead of just a single page.

We assume that the size of the anonymous folio chosen at allocation time
is still a good choice and therefore it is better to copy the entire
folio rather than a single page. It does not seem wise to do this for
file-backed folios, since the folio size chosen there is related to the
system-wide usage of the file. So we continue to CoW a single page for
file-backed mappings.

There are edge cases where the original mapping has been mremapped or
partially munmapped. In this case the source folio may not be naturally
aligned in the virtual address space. In this case, we CoW a power-of-2
portion of the source folio which is aligned. A similar effect happens
when allocation of a high order destination folio fails. In this case,
we reduce the order to 0 until we are successful.

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