userns: Disassociate user_struct from the user_namespace.

Modify alloc_uid to take a kuid and make the user hash table global.
Stop holding a reference to the user namespace in struct user_struct.

This simplifies the code and makes the per user accounting not
care about which user namespace a uid happens to appear in.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index e216e1e..898e973 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -27,7 +27,6 @@
 {
 	struct user_namespace *ns, *parent_ns = new->user_ns;
 	struct user_struct *root_user;
-	int n;
 
 	ns = kmem_cache_alloc(user_ns_cachep, GFP_KERNEL);
 	if (!ns)
@@ -35,11 +34,8 @@
 
 	kref_init(&ns->kref);
 
-	for (n = 0; n < UIDHASH_SZ; ++n)
-		INIT_HLIST_HEAD(ns->uidhash_table + n);
-
 	/* Alloc new root user.  */
-	root_user = alloc_uid(ns, 0);
+	root_user = alloc_uid(make_kuid(ns, 0));
 	if (!root_user) {
 		kmem_cache_free(user_ns_cachep, ns);
 		return -ENOMEM;