ion: Ensure pooled pages use proper free() function

In upstream change ecd32842
("ion: Handle the memory mapping correctly on x86")

pages allocated from the ion page pool may have special page table
attributes. These attributes must be unset before the page is returned
to the buddy system.

This issue was introduced by an incorrect cherry-pick of the above
change.

CRs-Fixed: 997730
Change-Id: Ia9339e78a4fa3f4734b6c75bb35d226f293dafb5
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>