RDMA/hns: Add a group interfaces for optimizing buffers getting flow

Currently, the code for getting umem and kmem buffers exist many files,
this patch adds a group interfaces to simplify the buffers getting flow.

Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>