RDMA/hns: Refactor the code of creating srq

Move the related codes of creating user srq and kernel srq into two
independent functions as well as remove some unused code and
simplifications.

Link: https://lore.kernel.org/r/1562593285-8037-3-git-send-email-oulijun@huawei.com
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>