RDMA/rtrs: Introduce destroy_cq helper

The same code snip used twice, to avoid duplicate, replace it with a
destroy_cq helper.

Link: https://lore.kernel.org/r/20210922125333.351454-6-haris.iqbal@ionos.com


Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>