x86/mm: remove the linear mapping of the p2m tables.

x86/mm: remove the linear mapping of the p2m tables.

Mapping the p2m into the monitor tables was an important optimization
on 32-bit builds, where it avoided mapping and unmapping p2m pages
during a walk.  On 64-bit it makes no difference -- see
http://old-list-archives.xen.org/archives/html/xen-devel/2010-04/msg00981.html
Get rid of it, and use the explicit walk for all lookups.

Signed-off-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>