fs: dlm: cleanup listen sock handling

This patch removes save_listen_callbacks() and add_listen_sock() as they
are only used once in lowcomms functionality. For shutdown lowcomms it's
not necessary to whole flush the workqueues to synchronize with
restoring the old sk_data_ready() callback. Only the listen con receive
work need to be cancelled. For each individual node shutdown we should be
sure that last ack was been transmitted which is done by flushing per
connection swork worker.

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