RDMA/hns: Refactor root BT allocation for MTR

mainline inclusion
from mainline-v5.12-rc1
commit 1f704d8c
category: bugfix
bugzilla: 174002
CVE:NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1f704d8cc07269f31daf9bdafe84882ad7596a2c

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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>
Signed-off-by: wangsirong <wangsirong@huawei.com>
Reviewed-by: ChunZhi Hu <huchunzhi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>