staging: ion: shrink page-pool by page unit

This patch shrink page-pool by page unit.

The system shrinker calls ion_heap_shrink_count() to get nr_to_scan,
and pass it to ion_heap_shrink_scan().
The problem is the return value of ion_heap_shrink_count() is the number
of pages but ion_system_heap_shrink(), which is called by
ion_heap_shrink_scan(), gets the number of chunk.

The main root of this is that ion_page_pool_shrink() returns page count
via ion_page_pool_total() if it have to check pool size. But it frees
chunks of pages if it have to free pools.

This patch first fix ion_page_pool_shrink() to count only pages,
not chunks. And then ion_system_heap_shrink() to work on pages.

Signed-off-by: Gioh Kim <gioh.kim@lge.com>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>