Fix chattr/cleanup failure

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

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6749

tests/zfs-tests/include/libtest.shlib

@@ -2255,7 +2255,7 @@ function del_user #<logname> <basedir>
 	fi
 
 	if id $user > /dev/null 2>&1; then
-		log_must userdel $user
+		log_must_retry "currently used" 5 userdel $user
 	fi
 
 	[[ -d $basedir/$user ]] && rm -fr $basedir/$user