staging: ion: fixup invalid kfree() calls on heap destroy

I've noticed that the last commit to ion_system_heap.c ('staging: ion:
optimize struct ion_system_heap') has an omission, so an invalid kfree()
gets called on ion_system_heap_destroy(). As ION system heap is never
destroyed until system shutdown, it may not cause any harm, but should
be fixed. I should have caught this before the merge, my bad.

Signed-off-by: Heesub Shin <heesub.shin@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>