RDMA/hns: Refactor root BT allocation for MTR

[ Upstream commit 1f704d8c ]

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.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>
Stable-dep-of: d586628b ("RDMA/hns: Fix the overflow risk of hem_list_calc_ba_range()")
Signed-off-by: Sasha Levin <sashal@kernel.org>