ocfs2: Move the call of ocfs2_hb_ctl into the stack glue.

Take o2hb_stop() out of the o2cb code and make it part of the generic
stack glue as ocfs2_leave_group().  This also allows us to remove the
ocfs2_get_hb_ctl_path() function - everything to do with hb_ctl is now
part of stackglue.c.  o2cb no longer needs a ->hangup() function.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>