The "vma->vm_pgoff" variable is an unsigned long so if it's larger than
INT_MAX then "index" can be negative leading to an underflow.  Fix this
by changing the type of "index" to "unsigned long".

Fixes: ddd89d0a ("vhost_vdpa: support doorbell mapping via mmap")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200610085852.GB5439@mwandaSigned-off-by: Michael S. Tsirkin <mst@redhat.com>