LU-13508 mdc: chlg device could be used after free

There are some issue of the usage of dynamic devices used by
the changelog in MDC, which could cause the device to be used
after it is freed.

Change-Id: Iacf6fa7c8b612f1a373091cf88e7082c4860cfe4
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/38658
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>