x86: remove use of per-domain lock from page table entry handling

x86: remove use of per-domain lock from page table entry handling

This change results in a 5% performance improvement for kernel builds
on dual-socket quad-core systems (which is what I used for reference
for both 32- and 64-bit). Along with that, the amount of time reported
as spent in the kernel gets reduced by almost 25% (the fraction of
time spent in the kernel is generally reported significantly higher
under Xen than with a native kernel).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>