lvmetad: fix heap memory leak

lvmetad: fix heap memory leak

error could be reproduced by calling pvs periodically:
    #!/bin/bash
    while :
    do
        pvs
        sleep 1
    done

use top command to watch RES memory of lvmetad. After a few minutes,
its RES memory will grow for a few KB. Then stop calling pvs, while
its RES will not decrease.

This is because, when lvmetad make reponse for clent request, it
will malloc new chunk for s->vgid_to_metadata, while actually the
new chunk should be added to the reponse dm_config_tree, or it will
make the chunk list of s->vgid_to_metadata keep growing.

Signed-off-by: wangjufeng <wangjufeng@huawei.com>
Reviewed-by: liuzhiqiang <liuzhiqiang26@huawei.com>
Reviewed-by: guiyao <guiyao@huawei.com>