Some magic numbers are hard to understand, so replace them with macros or
add some comments for them.

Link: https://lore.kernel.org/r/20200126145504.9700-1-liweihang@huawei.comSigned-off-by: Lang Cheng <chenglang@huawei.com>
Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Wenpeng Liang <liangwenpeng@huawei.com>
Signed-off-by: Yixing Liu <liuyixing1@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>