dmaengine: fix dmaengine_unmap failure

The count which is used to get_unmap_data maybe not the same as the
count computed in dmaengine_unmap which causes to free data in a
wrong pool.

This patch fixes this issue by keeping the map count with unmap_data
structure and use this count to get the pool.

Cc: <stable@vger.kernel.org>
Signed-off-by: Xuelin Shi <xuelin.shi@freescale.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>