nfsd: silence sparse warning about accessing credentials

sparse says:

    fs/nfsd/auth.c:31:38: warning: incorrect type in argument 1 (different address spaces)
    fs/nfsd/auth.c:31:38:    expected struct cred const *cred
    fs/nfsd/auth.c:31:38:    got struct cred const [noderef] <asn:4>*real_cred

Add a new accessor for the ->real_cred and use that to fetch the
pointer. Accessing current->real_cred directly is actually quite safe
since we know that they can't go away so this is mostly a cosmetic fixup
to silence sparse.
Signed-off-by: NJeff Layton <jlayton@primarydata.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/auth.c

@@ -28,7 +28,7 @@ int nfsd_setuser(struct svc_rqst *rqstp, struct svc_export *exp)
 	validate_process_creds();
 
 	/* discard any old override before preparing the new set */
-	revert_creds(get_cred(current->real_cred));
+	revert_creds(get_cred(current_real_cred()));
 	new = prepare_creds();
 	if (!new)
 		return -ENOMEM;

include/linux/cred.h

@@ -258,6 +258,15 @@ static inline void put_cred(const struct cred *_cred)
 #define current_cred() \
 	rcu_dereference_protected(current->cred, 1)
 
+/**
+ * current_real_cred - Access the current task's objective credentials
+ *
+ * Access the objective credentials of the current task.  RCU-safe,
+ * since nobody else can modify it.
+ */
+#define current_real_cred() \
+	rcu_dereference_protected(current->real_cred, 1)
+
 /**
  * __task_cred - Access a task's objective credentials
  * @task: The task to query