RDMA/hns: Fix the overflow risk of hem_list_calc_ba_range()

RDMA/hns: Fix the overflow risk of hem_list_calc_ba_range()

The max value of 'unit' and 'hop_num' is 2^24 and 2, so the value of
'step' may exceed the range of u32. Change the type of 'step' to u64.

Fixes: 38389eaa4db1 ("RDMA/hns: Add mtr support for mixed multihop addressing")
Signed-off-by: wenglianfa <wenglianfa@huawei.com>
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Link: https://patch.msgid.link/20240906093444.3571619-5-huangjunxian6@hisilicon.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>