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

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@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Tomasz Wroblewski <tomasz.wroblewski@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>