iommu/vt-d: Return error code in domain_context_mapping_one()

In 'commit <55d94043> ("iommu/vt-d: Get rid of domain->iommu_lock")',
the error handling path is changed a little, which makes the function
always return 0.

This path fixes this.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Fixes: 55d94043 ('iommu/vt-d: Get rid of domain->iommu_lock')
Cc: stable@vger.kernel.org # v4.3+
Signed-off-by: Joerg Roedel <jroedel@suse.de>