ocfs2/cluster: Pin the local node when o2hb thread starts

The patch pins the node item of the local node when the o2hb thread
starts and unpins on stop.

An earlier patch pinned the node item of the remote node on o2net
connect and unpinned on disconnect.

Signed-off-by Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>