x86: Retry do_mmu_update() a few times when called on a pte whose type is in flux.

x86: Retry do_mmu_update() a few times when called on a pte whose type is in flux.

This can really happen -- all our PV Linux kernels have a race
between vmalloc_sync_all() and pgdir pinning/unpinning. The former is
protected by pgd_lock while the latter by mm->page_table_lock. Hence
they can happen concurrently, and vmalloc_sync_all() can attempt to
set_pmd() on a page directory which is in the process of being
pinned. This can confuse the hypervisor which may see a type change,
and hence fail do_mmu_update(). Until this patch. :-)

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>