rpc_pipe: convert back to simple_dir_inode_operations

Now that Al has fixed simple_lookup to account for the case where
sb->s_d_op is set, there's no need to keep our own special lookup op.

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