DLM: fix overflow dlm_cb_seq

dlm_cb_seq is 64 bits. If dlm_cb_seq overflows and returns to 0,
dlm_rem_lkb_callback() will not work properly.

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>