iommu/debug: Add mutex to make attach/detach thread safe

This mutex lock will help to synchronise the usage of iommu domain
structure which is getting used by multiple threads to perform
iommu attach and iommu detach in case of any error.

Without this change we are seeing page poisoning - write after free
when running stress tests using iommu-debug test cases.

Change-Id: I388a90084ab8cc7e7097bac9a41ed5fed6dad312
Acked-by: Ankur Saxena <c_ankusa@qti.qualcomm.com>
Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Signed-off-by: VENKATA RAO KAKANI <vkakani@codeaurora.org>