vdpa/mlx5: workaround VAR offset within page

vdpa/mlx5: workaround VAR offset within page

vDPA driver first uses kernel driver to allocate doorbell (VAR) area for
each device. Then uses var->mmap_off and var->length to mmap uverbs device
file as doorbell userspace virtual address.

Current kernel driver provides var->mmap_off equal to page start of VAR.
It's fine with x86 4K page server, because VAR physical address is only 4K
aligned thus locate in 4K page start.

But with aarch64 64K page server, the actual VAR physical address has
offset within page (not located in 64K page start).
So the vDPA driver needs to add this within page offset
(caps.doorbell_bar_offset) to get the right VAR virtual address.

Fixes: 62c813706e4 ("vdpa/mlx5: map doorbell")
Cc: stable@dpdk.org
Signed-off-by: Yajun Wu <yajunw@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>