LU-16796 ptlrpc: Change struct ptlrpc_request_set to use kref

LU-16796 ptlrpc: Change struct ptlrpc_request_set to use kref

This patch changes struct ptlrpc_request_set to use
kref instead of atomic_t

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: Icd8dc9d532121b9087455b951a1b7ee922ab532c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53459
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Timothy Day <timday@amazon.com>