RDMA/hns: Refactor mtr find

RDMA/hns: Refactor mtr find

[ Upstream commit a4ca341080758d847db155b97887bff6f84016a4 ]

hns_roce_mtr_find() is a collection of multiple functions, and the
return value is also difficult to understand, which is not conducive
to modification and maintenance.

Separate the function of obtaining MTR root BA from this function.
And some adjustments has been made to improve readability.

Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Link: https://lore.kernel.org/r/20240113085935.2838701-2-huangjunxian6@hisilicon.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Stable-dep-of: 8673a6c2d9e4 ("RDMA/hns: Fix mapping error of zero-hop WQE buffer")
Signed-off-by: Sasha Levin <sashal@kernel.org>