ion: Fix two small issues in system_heap allocation

ion: Fix two small issues in system_heap allocation

In testing ion system heap allocations, I ran across two issues:

1) Not k*z*allocing the sg table. This can cause trouble if
we end up trying call sg_alloc_table() with too many entries,
then sg_alloc_table() internally fails and tries to free what it
thinks is internal table structure, which causes bad pointer
traversals.

2) The second list_for_each_entry probably should be _safe,
since I was seeing  strange lock warnings and oopses on occasion.
This seems to resolve it, but could use some extra checking.

Change-Id: I59d4c90104a8cf23dc4ae814d0b17348f1b68ac0
Signed-off-by: John Stultz <john.stultz@linaro.org>