gpu: ion: Fix bug in zeroing pages in system heap

Pages are zeroed for security purposes when returned to the
ion heap.  There was a bug in this code preventing this
from happening.

Bug: 7573871
Change-Id: I2b363924b7a352a4a8170a78777c01a04ac3f0ed
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: 78290a298e0659e016d9baae186dc8faed0a0382
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Laura Abbott <lauraa@codeaurora.org>