ANDROID: KVM: arm64: iommu: Free memory on registration error

Memory for IOMMU device entries gets allocated from a pool donated by
the host. It is possible for pkvm_iommu_register() to allocate the
memory and then fail, in which case the memory remains unused but not
freed.

Refactor the code such that the host lock covers the entire section
where the memory is allocated. This way we can return the memory back to
the linear allocator if an error is returned.

Bug: 190463801
Signed-off-by: David Brazdil <dbrazdil@google.com>
Change-Id: I8c1650ba3e545741144d793de506e93c4066896f