userns: Implement sk_user_ns

Add a helper sk_user_ns to make it easy to find the user namespace
of the process that opened a socket.
Acked-by: NDavid S. Miller <davem@davemloft.net>
Acked-by: NSerge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>

include/net/sock.h

@@ -604,6 +604,15 @@ static inline void sk_add_bind_node(struct sock *sk,
 #define sk_for_each_bound(__sk, node, list) \
 	hlist_for_each_entry(__sk, node, list, sk_bind_node)
 
+static inline struct user_namespace *sk_user_ns(struct sock *sk)
+{
+	/* Careful only use this in a context where these parameters
+	 * can not change and must all be valid, such as recvmsg from
+	 * userspace.
+	 */
+	return sk->sk_socket->file->f_cred->user_ns;
+}
+
 /* Sock flags */
 enum sock_flags {
 	SOCK_DEAD,