gnttab: don't blindly free status pages upon version change

gnttab: don't blindly free status pages upon version change

There may still be active mappings, which would trigger the respective
BUG_ON(). Split the loop into one dealing with the page attributes and
the second (when the first fully passed) freeing the pages. Return an
error if any pages still have pending references.

This is part of XSA-255.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 38bfcc165dda5f4284d7c218b91df9e144ddd88d
master date: 2018-02-27 14:07:12 +0100