ion: free low memory from page pools first

When the shrinkers are called with GFP_HIGH free low memory first,
it is more important to have free than high memory.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>