rpc_pipefs: clear write bit from top level rpc_pipefs directory

We can't create new files or directories here from userspace, so let's
not pretend that this directory is writable.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>