xen: Clear PG_pinned in release_{pt,pd}()

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Cc: xen-devel@lists.xensource.com
Cc: Mark McLoughlin <markmc@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>