This allows to make decision to drop a page right after writeout for
wcfs case. If vmas for one fileh were allowed to be mixed - we could not
know whether to drop the page & its memory from RAM or not - even if
there is no vma mapped in !wcfs mode, it could be created later, and by
dropping RAM cahce we hit severe slowness. NOTE: in between transactions
there is usually no vmas mapped, so this case is not artificial.

By requiring that all VMAs are of the same kind under one fileh we avoid
this ambiguity.