arm64/iommu: Fix a resource leak in smmu_domain_alloc()

We should free the allocated ASID if smmu_init_cd() fails.

Move the allocation of "domain" to simplify the first error path.

Reported by: Kevin Day <kevin@your.org>
Reviewed by: br
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54676