x86/xen: correct bug in p2m list initialization

x86/xen: correct bug in p2m list initialization

Commit 8e0f8ed37c60faa0a62b7d951fd4b90882a794cf ("xen: switch to
linear virtual mapped sparse p2m list") introduced an error.

During initialization of the p2m list a p2m identity area mapped by
a complete identity pmd entry has to be split up into smaller chunks
sometimes, if a non-identity pfn is introduced in this area.

If this non-identity pfn is not at index 0 of a p2m page the new
p2m page needed is initialized with wrong identity entries, as the
identity pfns don't start with the value corresponding to index 0,
but with the initial non-identity pfn. This results in weird wrong
mappings.

Correct the wrong initialization by starting with the correct pfn.

Cc: stable@vger.kernel.org # 3.19
Reported-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>