From: Hugh Dickins <hugh@veritas.com>

Why should try_to_unmap_anon and try_to_unmap_file take a copy of
page->mapcount and pass it down for try_to_unmap_one to decrement?  why not
just check page->mapcount itself?  asks akpm.  Perhaps there used to be a good
reason, but not any more: remove the mapcount arg.