gnttab: fix pin count / page reference race

gnttab: fix pin count / page reference race

Dropping page references before decrementing pin counts is a bad idea
if assumptions are being made that a non-zero pin count implies a valid
page. Fix the order of operations in gnttab_copy_release_buf(), but at
the same time also remove the assertion that was found to trigger:
map_grant_ref() also has the potential of causing a race here, and
changing the order of operations there would likely be quite a bit more
involved.

This is CVE-2017-15597 / XSA-236.

Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>