SUNRPC: Add a downcall queue to struct rpc_inode

Currently, the downcall queue is tied to the struct gss_auth, which means
that different RPCSEC_GSS pseudoflavours must use different upcall pipes.
Add a list to struct rpc_inode that can be used instead.

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

include/linux/sunrpc/rpc_pipe_fs.h

@@ -23,6 +23,7 @@ struct rpc_inode {
 	void *private;
 	struct list_head pipe;
 	struct list_head in_upcall;
+	struct list_head in_downcall;
 	int pipelen;
 	int nreaders;
 	int nwriters;