iommu/vt-d: Remove misleading "domain 0" test from domain_exit()

The "Domain 0 is reserved, so dont process it" comment suggests that a NULL
pointer corresponds to domain 0.  I don't think that's true, and in any
case, every caller supplies a non-NULL domain pointer that has already been
dereferenced, so the test is unnecessary.

Remove the test for a null "domain" pointer.  No functional change
intended.

This null pointer check was added by 5e98c4b1 ("Allocation and free
functions of virtual machine domain").
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel-iommu.c

@@ -1903,10 +1903,6 @@ static void domain_exit(struct dmar_domain *domain)
 {
 	struct page *freelist;
 
-	/* Domain 0 is reserved, so dont process it */
-	if (!domain)
-		return;
-
 	/* Remove associated devices and clear attached or cached domains */
 	rcu_read_lock();
 	domain_remove_dev_info(domain);