PCI: Use vma_pages() to replace (vm_end - vm_start) >> PAGE_SHIFT

(*->vm_end - *->vm_start) >> PAGE_SHIFT operation is implemented
as an inline funcion vma_pages() in linux/mm.h, so use it.

Signed-off-by: Libin <huawei.libin@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>