userns: Use cred->user_ns instead of cred->user->user_ns

Optimize performance and prepare for the removal of the user_ns reference
from user_struct.  Remove the slow long walk through cred->user->user_ns and
instead go straight to cred->user_ns.

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 3b906e9..f084083 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -90,7 +90,7 @@
 {
 	struct user_namespace *tmp;
 
-	if (likely(to == cred->user->user_ns))
+	if (likely(to == cred->user_ns))
 		return uid;
 
 
@@ -112,7 +112,7 @@
 {
 	struct user_namespace *tmp;
 
-	if (likely(to == cred->user->user_ns))
+	if (likely(to == cred->user_ns))
 		return gid;
 
 	/* Is cred->user the creator of the target user_ns