Fix a device pager leak for the case where the page already exists in the

Fix a device pager leak for the case where the page already exists in the
VM object (typical case: multiple mappings of the device?).  If the page
already exists we simply update its physical address.  It is unclear whether
the physical address would ever actually be different, however.

This is an untested patch.

Original-patch-written-by: Christian Zander @ NVIDIA
Workaround-suggested-by: Tor Egge <tegge@freebsd.org>
Submitted-by: Emiel Kollof <coolvibe@hackerheaven.org>