Fix chattr/cleanup failure

[behlendorf]

Description

The chattr cleanup step may fail to delete the user if there is still
an active process running as that user. Retry the userdel when this
occurs to eliminate spurious false positves.

ERROR: userdel quser1 exited 8
userdel: user quser1 is currently used by process 26814

Motivation and Context

Resolve occasional automated test failures like this.

How Has This Been Tested?

Locally by running the chattr tests in a loop.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[behlendorf]

Fedora 26 seems to hit this pretty frequently, the retry in the patch was verified to address this issue. Test log.

Test: /usr/share/zfs/zfs-tests/tests/functional/chattr/cleanup (run as root) [00:01] [PASS]
23:10:04.80 ERROR: userdel quser1 Retry in 1 seconds
23:10:05.88 SUCCESS: userdel quser1
23:10:05.89 SUCCESS: del_user quser1
23:10:06.04 SUCCESS: userdel quser2
23:10:06.05 SUCCESS: del_user quser2
23:10:06.11 SUCCESS: groupdel qgroup
23:10:06.12 SUCCESS: del_group qgroup
23:10:06.13 SUCCESS: clean_user_group
23:10:06.26 SUCCESS: zpool destroy -f testpool
23:10:06.28 SUCCESS: rm -rf /mnt/testdir