RDMA/hns: Fix init resp when alloc ucontext

The data in resp will be copied from kernel to userspace, thus it needs to
be initialized to zeros to avoid copying uninited stack memory.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e088a685 ("RDMA/hns: Support rq record doorbell for the user space")
Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>