[PATCH] Fix lazy mode vmalloc synchronization for paravirt

Found this looping Ubuntu installs with VMI.

If unlucky enough to hit a vmalloc sync fault during a lazy mode operation (from an IRQ handler for a module which was not yet populated in current page directory, or from inside copy_one_pte, which touches swap_map, and hit in an unused 4M region), the required PDE update would never get flushed, causing an infinite page fault loop.

This bug affects any paravirt-ops backend which uses lazy updates, I believe that makes it a bug in Xen, VMI and lguest.  It only happens on LOWMEM kernels.

Currently for 2.6.23, but we'll want to backport to -stable as well.

Zach