6843 Make xattr dir truncate and remove in one tx

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Chunwei Chen <tuxoko@gmail.com>

openzfs/openzfs@399cc7d5d9aff97c714b708af3e3f0280ceab93f