Encapsulate the process of obtaining the current QP and filling WC as
functions, also merge some duplicate code.

Link: https://lore.kernel.org/r/1616815294-13434-2-git-send-email-liweihang@huawei.comSigned-off-by: NWeihang Li <liweihang@huawei.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>