cred: Refcount the user_ns pointed to by the cred.

struct user_struct will shortly loose it's user_ns reference
so make the cred user_ns reference a proper reference complete
with reference counting.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

include/linux/cred.h

@@ -146,7 +146,7 @@ struct cred {
 	void		*security;	/* subjective LSM security */
 #endif
 	struct user_struct *user;	/* real user ID subscription */
-	struct user_namespace *user_ns; /* cached user->user_ns */
+	struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
 	struct group_info *group_info;	/* supplementary groups for euid/fsgid */
 	struct rcu_head	rcu;		/* RCU deletion hook */
 };