ocfs2: Replace panic() with emergency_restart() when fencing

We have noticed panic() hanging leading us to a situation in which
the node, while otherwise dead, is still disk heartbeating. This
leads to a hung cluster as the other nodes are waiting for this
node to stop disk heartbeating. This situation is only resolved
by power resetting the box.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>