zap_pte_range() is currently just dropping the pte.  Change it to mark
the page referenced if the pte says it was.  This has the effect of
delaying the eviction of recently-mapped pagecache.

This means that we're currently marking the page accessed when it is
first faulted in as well as when we drop it from pagetables.  Which
matches up with the (strange) behaviour of the VM: it reclaims
PageReferenced pagecache pages off the inactive list.

Probably, it makes sense to remove the mark_page_accessed() from
filemap_nopage() and just use the pte bits everywhere.  Reviewing all
the PageReferenced()/mark_page_accessed() usage is on my todo list.