x86/mm: Ensure useful progress in alloc_l2_table()

While debugging the issue which turned out to be XSA-58, a printk in this loop
showed that it was quite easy to never make useful progress, because of
consistently failing the preemption check.

One single l2 entry is a reasonable amount of work to do, even if an action is
pending, and also assures forwards progress across repeat continuations.

Tweak the continuation criteria to fail on the first iteration of the loop.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>

xen/arch/x86/mm.c

@@ -1278,7 +1278,8 @@ static int alloc_l2_table(struct page_info *page, unsigned long type,
 
     for ( i = page->nr_validated_ptes; i < L2_PAGETABLE_ENTRIES; i++ )
     {
-        if ( preemptible && i && hypercall_preempt_check() )
+        if ( preemptible && i > page->nr_validated_ptes
+             && hypercall_preempt_check() )
         {
             page->nr_validated_ptes = i;
             rc = -EAGAIN;