fs: dlm: be sure to call dlm_send_queue_flush()

If we release a midcomms node structure, there should be nothing left
inside the dlm midcomms send queue. However, sometimes this is not true
because I believe some DLM_FIN message was not acked... if we run
into a shutdown timeout, then we should be sure there is no pending send
dlm message inside this queue when releasing midcomms node structure.

Cc: stable@vger.kernel.org
Fixes: 489d8e55

 ("fs: dlm: add reliable connection if reconnect")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>