RDMA/hns: Optimize hns buffer allocation flow

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 and
understand.

Link: https://lore.kernel.org/r/1586779091-51410-3-git-send-email-liweihang@huawei.com


Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Lang Cheng <chenglang@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>