RDMA/hns: Refactor the hns_roce_buf allocation flow

Add a group of flags to control the 'struct hns_roce_buf' allocation
flow, this is used to support the caller running in atomic context.

Link: https://lore.kernel.org/r/1605347916-15964-1-git-send-email-liweihang@huawei.com


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