mm: exporting account_page_dirty

This allows code outside of the mm core to safely manipulate page state
and not worry about the other accounting. Not using these routines means
that some code will lose track of the accounting and we get bugs. This
has happened once already.

Signed-off-by: Michael Rubin <mrubin@google.com>
Signed-off-by: Sage Weil <sage@newdream.net>