xen/mmu: some early pagetable cleanups

1. make sure early-allocated ptes are pinned, so they can be later
   unpinned
2. don't pin pmd+pud, just make them RO
3. scatter some __inits around

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

diff --cc arch/x86/xen/mmu.c
Simple merge

diff --cc arch/x86/xen/xen-ops.h
Simple merge