RDMA/hns: Add detailed comments for mb() call

RDMA/hns: Add detailed comments for mb() call

BugLink: http://bugs.launchpad.net/bugs/1756097
This patch adds more detailed comments when we call the
memory barrier function, such as rmb, wmb and mb. Three
mb() callers are deleted since they are unnecessary.

Suggested-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
(cherry picked from commit 4044a3f482a3373ea5379da47c04ebecb9a3f133)
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>