RDMA/hns: Refactor root BT allocation for MTR

Split the hem_list_alloc_root_bt() into serval small functions to make the
code flow more clear.

Link: https://lore.kernel.org/r/1621589395-2435-3-git-send-email-liweihang@huawei.comSigned-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>