DLM: fix to use sk_callback_lock correctly

In the current implementation, we think that exclusion control between
processing to set the callback function to the connection structure and
processing to refer to the connection structure from the callback function
was not enough. We fix them.

Signed-off-by: Tadashi Miyauchi <miyauchi@toshiba-tops.co.jp>
Signed-off-by: Tsutomu Owa <tsutomu.owa@toshiba.co.jp>
Signed-off-by: David Teigland <teigland@redhat.com>