6842 Fix empty xattr dir causing lockup

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@02525cd08fb3730fff3a69cb5376443d481f7839