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>

fs/ocfs2/cluster/heartbeat.c

@@ -2389,6 +2389,9 @@ static int o2hb_region_pin(const char *region_uuid)
 	assert_spin_locked(&o2hb_live_lock);
 
 	list_for_each_entry(reg, &o2hb_all_regions, hr_all_item) {
+		if (reg->hr_item_dropped)
+			continue;
+
 		uuid = config_item_name(&reg->hr_item);
 
 		/* local heartbeat */
@@ -2439,6 +2442,9 @@ static void o2hb_region_unpin(const char *region_uuid)
 	assert_spin_locked(&o2hb_live_lock);
 
 	list_for_each_entry(reg, &o2hb_all_regions, hr_all_item) {
+		if (reg->hr_item_dropped)
+			continue;
+
 		uuid = config_item_name(&reg->hr_item);
 		if (region_uuid) {
 			if (strcmp(region_uuid, uuid))
@@ -2654,6 +2660,9 @@ int o2hb_get_all_regions(char *region_uuids, u8 max_regions)
 
 	p = region_uuids;
 	list_for_each_entry(reg, &o2hb_all_regions, hr_all_item) {
+		if (reg->hr_item_dropped)
+			continue;
+
 		mlog(0, "Region: %s\n", config_item_name(&reg->hr_item));
 		if (numregs < max_regions) {
 			memcpy(p, config_item_name(&reg->hr_item),