x86: don't use destroy_xen_mappings() for vunmap()

x86: don't use destroy_xen_mappings() for vunmap()

Its attempt to tear down intermediate page table levels may race with
map_pages_to_xen() establishing them, and now that
map_domain_page_global() is backed by vmap() this teardown is also
wasteful (as it's very likely to need the same address space populated
again within foreseeable time).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
master commit: 68caac7f6f4687241a24e804a9fca19aa26fe183
master date: 2013-07-17 10:21:33 +0200