LU-16796 mdt: Change struct cdt_agent_req to use kref

LU-16796 mdt: Change struct cdt_agent_req to use kref

This patch changes struct cdt_agent_req to use
kref(refcount_t) instead of atomic_t

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I0a99002504ff453b8b748391f08bd1020c545321
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52148
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>