iommu/arm-smmu: Free context bitmap in the err path of arm_smmu_init_domain_context

When alloc_io_pgtable_ops is failed, context bitmap which is just allocated
by __arm_smmu_alloc_bitmap should be freed to release the resource.

Signed-off-by: Liu Xiang <liuxiang_1999@126.com>
Signed-off-by: Will Deacon <will@kernel.org>