SUNRPC: Fix a memory leak in the auth credcache code

The leak only affects the RPCSEC_GSS caches, since they are the only ones
that are dynamically allocated...
Rename the existing rpcauth_free_credcache() to rpcauth_clear_credcache()
in order to better describe its role, then add a new function
rpcauth_destroy_credcache() that actually frees the cache in addition to
clearing it out.

Also move the call to destroy the credcache in gss_destroy() to come before
the rpc upcall pipe is unlinked.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
index 9527f2b..f6b6c81 100644
--- a/net/sunrpc/auth.c
+++ b/net/sunrpc/auth.c
@@ -137,9 +137,8 @@
  * that are not referenced.
  */
 void
-rpcauth_free_credcache(struct rpc_auth *auth)
+rpcauth_clear_credcache(struct rpc_cred_cache *cache)
 {
-	struct rpc_cred_cache *cache = auth->au_credcache;
 	HLIST_HEAD(free);
 	struct hlist_node *pos, *next;
 	struct rpc_cred	*cred;
@@ -157,6 +156,21 @@
 	rpcauth_destroy_credlist(&free);
 }
 
+/*
+ * Destroy the RPC credential cache
+ */
+void
+rpcauth_destroy_credcache(struct rpc_auth *auth)
+{
+	struct rpc_cred_cache *cache = auth->au_credcache;
+
+	if (cache) {
+		auth->au_credcache = NULL;
+		rpcauth_clear_credcache(cache);
+		kfree(cache);
+	}
+}
+
 static void
 rpcauth_prune_expired(struct rpc_auth *auth, struct rpc_cred *cred, struct hlist_head *free)
 {