SUNRPC: Don't spam gssd with upcall requests when the kerberos key expired

Now that the rpc.gssd daemon can explicitly tell us that the key expired,
we should cache that information to avoid spamming gssd.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/auth.h

@@ -54,6 +54,7 @@ struct rpc_cred {
 #define RPCAUTH_CRED_NEW	0
 #define RPCAUTH_CRED_UPTODATE	1
 #define RPCAUTH_CRED_HASHED	2
+#define RPCAUTH_CRED_NEGATIVE	3
 
 #define RPCAUTH_CRED_MAGIC	0x0f4aa4f0