ZTS: harden xattr/cleanup.ksh

When the xattr/cleanup.ksh script is unable to remove the test group
due to an active process then it will not call default_cleanup. This
will result in a zvol_ENOSPC/setup failure when attempting to create
the /mnt/testdir directory which will already exist.

Resolve the issue by performing the default_cleanup before removing
the test user and group to ensure this step always happens. Also
allow one more retry to further minimize the likelihood of the
cleanup failing.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9358