ion: carveout heap: zero buffers on free, fix memory leak

The carveout heap wasn't zeroing its buffers after use.
Create the sg_table during allocate instead of map_dma, to allow
using the sg_table during free, and call ion_heap_buffer_zero
during free.  Also fixes a missing kfree when destroying the
table.

Change-Id: I318a8493cce32580250884cae336dd2e2c28e73b
Signed-off-by: Colin Cross <ccross@android.com>