BugLink: https://launchpad.net/bugs/1839521

Sync ZFS 0.6.5.6-0ubuntu28 to Xenial:

Fix shrinker deadlock with xattrs (LP: #1839521)

Upstream ZFS fix 31b6111fd92a ("Kill zp->z_xattr_parent to prevent pinning")
and ddae16a9cf0b ("xattr dir doesn't get purged during iput") fix a deadlock
in shrinker path when a xattr directory inode and its xattr inode are in the
same disposal list and the xattr dir inode is evicted before the xattr inode.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>