vfio/type1: fix dirty bitmap calculation in vfio_dma_rw

The count of dirtied pages is not only determined by count of copied
pages, but also by the start offset.

e.g. if offset = PAGE_SIZE - 1, and *copied=2, the dirty pages count
is 2, instead of 1 or 0.

Fixes: d6a4c185 ("vfio iommu: Implementation of ioctl for dirty pages tracking")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>