iommu/arm-smmu: hold init_mutex in iommu_domain_get_attr

Some attributes are changed during domain attach and detach,
so hold init_mutex to ensure consistency.

Change-Id: I450a9a2da4bfe3616ef6dd0a6426271d25c292ce
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>