ocfs2: fix NULL pointer dereference when traversing o2hb_all_regions

There may exist NULL pointer dereference in config_item_name() when one
volume (say Volume A) unmounts while another (say Volume B) mounting.

     Volume A                          Volume B

  already Mounted.
  Unmounting, call
  o2hb_heartbeat_group_drop_item()
    -> config_item_put(item)
    set reg(A)->item.ci_name to NULL
    in function config_item_cleanup().

                                    begin mounting, call
                                    o2hb_region_pin() and tranverse all
                                    regions. When reading
                                    reg(A)->item.ci_name, it causes
                                    NULL pointer dereference.

  call o2hb_region_release() and
  del reg(A) from list.

So we should skip accessing regions that is going to release when
tranverse o2hb_all_regions.

Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com>
Signed-off-by: joyce <xuejiufei@huawei.com>
Acked-by: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Sunil Mushran <sunil.mushran@gmail.com>
Cc: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>