Complete r320189 which allows a NULL VM fault handler in the LinuxKPI.
Instead of mapping a dummy page upon a page fault, map the page
pointed to by the physical address given by IDX_TO_OFF(vmap->vm_pfn).
To simplify the implementation use OBJT_DEVICE to implement our own
linux_cdev_pager_fault() instead of using the existing
linux_cdev_pager_populate().

Some minor code factoring while at it.

Reviewed by: markj @
MFC after: 1 week
Sponsored by: Mellanox Technologies