intel-iommu: Fix enabling snooping feature by mistake

Two defects work together result in KVM device passthrough randomly can't
work:
1. iommu_snooping is not initialized to zero when vm_iommu_init() called.
So it is possible to get a random value.
2. One line added by commit 2c2e2c38("IOMMU Identity Mapping Support")
change the code path, let it bypass domain_update_iommu_cap(), as well as
missing the increment of domain iommu reference count.

The latter is also likely to cause a leak of domains on repeated VMM 
assignment and deassignment.
Signed-off-by: NSheng Yang <sheng@linux.intel.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

drivers/pci/intel-iommu.c

@@ -1505,7 +1505,6 @@ static int domain_context_mapping_one(struct dmar_domain *domain, int segment,
 			}
 
 			set_bit(num, iommu->domain_ids);
-			set_bit(iommu->seq_id, &domain->iommu_bmp);
 			iommu->domains[num] = domain;
 			id = num;
 		}
@@ -3409,6 +3408,7 @@ static int md_domain_init(struct dmar_domain *domain, int guest_width)
 
 	domain->iommu_count = 0;
 	domain->iommu_coherency = 0;
+	domain->iommu_snooping = 0;
 	domain->max_addr = 0;
 
 	/* always allocate the top pgd */