RDMA/hns: Refactor the alloc_srqc()

Abstract the alloc_srqc() into several parts and separate the alloc_srqn()
from the alloc_srqc().

Link: https://lore.kernel.org/r/20220302064830.61706-9-liangwenpeng@huawei.com


Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Wenpeng Liang <liangwenpeng@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>