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/security/keys/permission.c b/security/keys/permission.c
index c35b522..e146cbd 100644
--- a/security/keys/permission.c
+++ b/security/keys/permission.c
@@ -36,7 +36,7 @@
 
 	key = key_ref_to_ptr(key_ref);
 
-	if (key->user->user_ns != cred->user->user_ns)
+	if (key->user->user_ns != cred->user_ns)
 		goto use_other_perms;
 
 	/* use the second 8-bits of permissions for keys the caller owns */