mm: move_pages: no need to set pp->page to ZERO_PAGE(0) by default

pp->page is never used when not set to the right page, so there is no need
to set it to ZERO_PAGE(0) by default.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>