[DLM] fix socket shutdown

This patch clears the user_data of active sockets as part of cleanup.
This prevents any late-arriving data from trying to add jobs to the work
queue while we are tidying up.

Signed-Off-By: Patrick Caulfield <pcaulfie@redhat.com>
Signed-Off-By: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>