ocfs2/dlm: Add new dlm message DLM_BEGIN_EXIT_DOMAIN_MSG

This patch adds a new dlm message DLM_BEGIN_EXIT_DOMAIN_MSG and ups the dlm
protocol to 1.2.

o2dlm sends this new message in dlm_unregister_domain() to mark the beginning
of the exit domain. This message is sent to all nodes in the domain.

Currently o2dlm has no way of informing other nodes of its impending exit.
This information is useful as the other nodes could disregard the exiting
node in certain operations. For example, in resource migration. If two or
more nodes were umounting in parallel, it would be more efficient if o2dlm
were to choose a non-exiting node to be the new master node rather than an
exiting one.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Reviewed-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <jlbec@evilplan.org>