vfio: Fix PCI mmap after b3b9c293

Our mmap path mistakely relied on vma->vm_pgoff to get set in
remap_pfn_range.  After b3b9c293, that path only applies to
copy-on-write mappings.  Set it in our own code.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>