fb_defio: fix for non-dirty ptes

Fix a problem observed while using fb_defio with a short delay on a
PowerPC platform.

It is possible that page_mkclean() is invoked in the deferred io work
function _before_ a PTE has been marked dirty.  In this case, the page is
removed from the defio pagelist but page_mkclean() does not write-protect
the page again.  The end result is that defio ignores all subsequent
writes to the page and the corresponding portions of the framebuffer never
get updated.

The fix consists in keeping track of the pages with non-dirty PTEs,
re-checking them again on the next deferred io work iteration.  Note that
those pages are not passed to the defio callback as they are not written
by userspace yet.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Acked-by: Jaya Kumar <jayakumar.lkml@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>