dlm: fix no ack after final message

In case of an final DLM message we can't should not send an ack out
after the final message. This patch moves the ack message before the
messages will be transmitted. If it's the final message and the
receiving node turns into DLM_CLOSED state another ack messages will
being received and turning the receiving node into DLM_ESTABLISHED
again.

Fixes: 1696c75f ("fs: dlm: add send ack threshold and append acks to msgs")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>