RDMA/hns: Fix sg offset non-zero issue

RDMA/hns: Fix sg offset non-zero issue

When run perftest in many times, the system will report a BUG as follows:

   BUG: Bad rss-counter state mm:(____ptrval____) idx:0 val:-1
   BUG: Bad rss-counter state mm:(____ptrval____) idx:1 val:1

We tested with different kernel version and found it started from the the
following commit:

commit d10bcf947a3e ("RDMA/umem: Combine contiguous PAGE_SIZE regions in
SGEs")

In this commit, the sg->offset is always 0 when sg_set_page() is called in
ib_umem_get() and the drivers are not allowed to change the sgl, otherwise
it will get bad page descriptor when unfolding SGEs in __ib_umem_release()
as sg_page_count() will get wrong result while sgl->offset is not 0.

However, there is a weird sgl usage in the current hns driver, the driver
modified sg->offset after calling ib_umem_get(), which caused we iterate
past the wrong number of pages in for_each_sg_page iterator.

This patch fixes it by correcting the non-standard sgl usage found in the
hns_roce_db_map_user() function.

Fixes: d10bcf947a3e ("RDMA/umem: Combine contiguous PAGE_SIZE regions in SGEs")
Fixes: 0425e3e6e0c7 ("RDMA/hns: Support flush cqe for hip08 in kernel space")
Link: https://lore.kernel.org/r/1562808737-45723-1-git-send-email-oulijun@huawei.com
Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>