ocfs2: Check heartbeat mode for kernel stacks only

Commit 2c442719 added some checks for proper
heartbeat mode when the o2cb stack is running.  Unfortunately, it didn't
take into account that a userpsace stack could be running. Fix this by only
doing the check if o2cb is in use. This patch allows userspace stacks to
mount the fs again.

Cc: stable@kernel.org
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <jlbec@evilplan.org>