RPC: stops the release_pipe() funtion from being called twice

 This patch stops the release_pipe() funtion from being called
 twice by invalidating the ops pointer in the rpc_inode
 when rpc_pipe_release() is called.

 Signed-off-by: Steve Dickson <steved@redhat.com>
 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>