RDMA/hns: Optimize hns buffer allocation flow

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

When the value of nbufs is 1, the buffer is in direct mode, which may cause
confusion. So optimizes current codes to make it easier to maintain.
Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Shunfeng Yang <yangshunfeng2@huawei.com>
Reviewed-by: chunzhi hu <huchunzhi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>