Comment 4 for bug 1839521

Fix Details
-----------

Problem report (GH Issue) upstream in ZFS on Linux:
 https://github.com/zfsonlinux/zfs/issues/4816 ("Looping in zfs_zget()")

With very similar stack trace (syscall -> shrink -> zfs_{inactive/rmnode/purgedir/zget})
 https://github.com/zfsonlinux/zfs/issues/4816#issuecomment-229532796

Proposed patch in the Issue (not the right/final solution):
 https://github.com/zfsonlinux/zfs/issues/4816#issuecomment-229537558
 https://github.com/zfsonlinux/zfs/pull/4822 ("Allow zfs_purgedir() to skip inodes undergoing eviction")

However, that proposed patch is not the right/final solution:
 https://github.com/zfsonlinux/zfs/pull/4822#issuecomment-230938382
 https://github.com/zfsonlinux/zfs/pull/4822#issuecomment-232208224

Instead, these proposed patches are actually the right/final solution:
 https://github.com/zfsonlinux/zfs/pull/4827 ("xattr dir doesn't get purged during iput")

The two patches are merged upstream:
 https://github.com/zfsonlinux/zfs/pull/4827#issuecomment-232207013

 1) ddae16a xattr dir doesn't get purged during ipu
 2) 31b6111 Kill zp->z_xattr_parent to prevent pinning

Available in the 0.7.0 release:

 $ git describe --contains ddae16a 31b6111
 zfs-0.7.0-rc1~102
 zfs-0.7.0-rc1~101

Thus Ubuntu only needs these patches on Xenial (zfs-linux < 0.7.0):

 $ rmadison zfs-linux
  zfs-linux | 0.6.5.6-0ubuntu8 | xenial/universe | source
  zfs-linux | 0.6.5.6-0ubuntu27 | xenial-updates | source
  zfs-linux | 0.7.5-1ubuntu15 | bionic | source
  zfs-linux | 0.7.5-1ubuntu16.6 | bionic-updates | source
  zfs-linux | 0.7.12-1ubuntu5 | disco | source
  zfs-linux | 0.8.1-1ubuntu8 | eoan | source

There's a later commit that changes/"reverts" the
commit ddae16a ("xattr dir doesn't get purged during ipu"),
but it's not a bug fix, but rather a refactor/feature.

 https://github.com/zfsonlinux/zfs/pull/4838

With commit dfbc86309fd8 (below).

The commit ids are not mentioned in further commits upstream.

 $ git log --oneline --grep ddae16a ddae16a^..origin/master
 dfbc86309fd8 Use native inode->i_nlink instead of znode->z_links

 $ git log --oneline --grep 31b6111 ddae16a^..origin/master
 $