fs: dlm: use list_empty() to check last iteration

This patch will use list_empty(&ls->ls_cb_delay) to check for last list
iteration. In case of a multiply count of MAX_CB_QUEUE and the list is
empty we do a extra goto more which we can avoid by checking on
list_empty().

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>