LU-11626 mdc: hold obd while processing changelog

LU-11626 mdc: hold obd while processing changelog

During read/write changelog, the corresponding obd_device should
be held to protect it from being released by umount.

Lustre-change: https://review.whamcloud.com/35784
Lustre-commit: d7bb6647cd4dd26949bceb6a099cd606623aff2b

Change-Id: Ib5b528f178edcf73425587ea60335df640c1696d
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36338
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>