Recent i686 2.6.27 kernels with a certain amount of memory (between 736 and
855MB) have a problem booting under a hypervisor that supports batched mprotect
(this includes the RHEL-5 Xen hypervisor as well as any 3.3 or later Xen
hypervisor).  The problem ends up being that xen_ptep_modify_prot_commit() is
using virt_to_machine to calculate which pfn to update.  However, this only
works for pages that are in the p2m list, and the pages coming from
change_pte_range() in mm/mprotect.c are kmap_atomic pages.  Because of this, we
can run into the situation where the lookup in the p2m table returns an
INVALID_MFN, which we then try to pass to the hypervisor, which then (correctly)
denies the request to a totally bogus pfn.

The right thing to do is to use arbitrary_virt_to_machine, so that we can be
sure we are modifying the right pfn.  This unfortunately introduces a
performance penalty because of a full page-table-walk, but we can avoid that
penalty for pages in the p2m list by checking if virt_addr_valid is true, and if
so, just doing the lookup in the p2m table.

The attached patch implements this, and allows my 2.6.27 i686 based guest with
768MB of memory to boot on a RHEL-5 hypervisor again.  Thanks to Jeremy for the
suggestions about how to fix this particular issue.

Signed-off-by: Chris Lalancette <clalance@redhat.com>