xmalloc: make whole pages xfree() clear the order field (ab)used by xmalloc()

Not doing this was found to cause problems with sequences of allocation
(multi-page), freeing, and then again allocation of the same page upon
boot when interrupts are still disabled (causing the owner field to be
non-zero, thus making the allocator attempt a TLB flush and, in its
processing, triggering an assertion).

Reported-by: Tomasz Wroblewski <tomasz.wroblewski@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Tomasz Wroblewski <tomasz.wroblewski@xxxxxxxxxx>

--- a/xen/common/xmalloc_tlsf.c
+++ b/xen/common/xmalloc_tlsf.c
@@ -629,6 +629,7 @@ void xfree(void *p)
         unsigned int i, order = get_order_from_pages(size);
 
         BUG_ON((unsigned long)p & ((PAGE_SIZE << order) - 1));
+        PFN_ORDER(virt_to_page(p)) = 0;
         for ( i = 0; ; ++i )
         {
             if ( !(size & (1 << i)) )