let __dec_zone_page_state use __dec_zone_state

This removes code duplication and makes __dec_zone_page_state look like
__inc_zone_page_state.

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>