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).

As the race between vmap() and vunmap(), according to the latest stage
tester logs, doesn't appear to be the only one still left, the patch
also adds logging for vmap() and vunmap() uses (there shouldn't be too
many of them, so logs shouldn't get flooded). These are supposed to
get removed (and are made stand out clearly) as soon as we're certain
that there's no issue left.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>