DLM: Save and restore socket callbacks properly

This patch fixes the problems with patch b3a5bbfd.

1. It removes a return statement from lowcomms_error_report
   because it needs to call the original error report in all paths
   through the function.
2. All socket callbacks are saved and restored, not just the
   sk_error_report, and that's done so with proper locking like
   sunrpc does.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>