RDMA/hns: Use the reserved loopback QPs to free MR before destroying MPT

Before destroying MPT, the reserved loopback QPs send loopback IOs (one
write operation per SL). Completing these loopback IOs represents that
there isn't any outstanding request in MPT, then it's safe to destroy MPT.

Link: https://lore.kernel.org/r/20220310042835.38634-1-liangwenpeng@huawei.com
Signed-off-by: Yixing Liu <liuyixing1@huawei.com>
Signed-off-by: Wenpeng Liang <liangwenpeng@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>