fs: dlm: synchronize dlm before shutdown

This patch moves the dlm workqueue dlm synchronization before shutdown
handling. The patch just flushes all pending work before starting to
shutdown the connection. At least for the send_workqeue we should flush
the workqueue to make sure there is no new connection handling going on
as dlm_allow_conn switch is turned to false before.

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