LU-12506 changelog: support large number of MDT

LU-12506 changelog: support large number of MDT

At client, the changelog of each MDT is associated with
one miscdevice, but the number of miscdevice is limited
to 64 in Linux kernel, then it will fail if there are
more than 64 MDTs.

This patch replaces miscdevice with dynamic devices to
support more MDTs.

Change-Id: Ie3ce76cbe1c613bf17d6350ea95546524b6d66b8
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/37759
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>