xen: handle highmem pages correctly when shrinking a domain

Commit 1058a75f ("xen: actually release
memory when shrinking domain") causes a crash if the page being released
is a highmem page.

If a page is highmem then there is no need to unmap it.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>