Since IOMMU map entries store a reference to the domain in which they reside, there is no need to pass the domain to iommu_gas_free_entry(), iommu_gas_free_space(), and iommu_gas_free_region().

Push down the acquisition and release of the IOMMU domain lock into iommu_gas_free_space() and iommu_gas_free_region().

Both of these changes allow for simplifications in the callers of the functions without really complicating the functions themselves. Moreover, the latter change eliminates the direct use of the IOMMU domain lock from the x86-specific DMAR code.