gnttab: fix unmap pin accounting race

gnttab: fix unmap pin accounting race

Once all {writable} mappings of a grant entry have been unmapped, the
hypervisor informs the guest that the grant entry has been released by
clearing the _GTF_{reading,writing} usage flags in the guest's grant
table as appropriate.

Unfortunately, at the moment, the code that updates the accounting
happens in a different critical section than the one which updates the
usage flags; this means that under the right circumstances, there may be
a window in time after the hypervisor reported the grant as being free
during which the grant referee still had access to the page.

Move the grant accounting code into the same critical section as the
reporting code to make sure this kind of race can't happen.

This is part of XSA-218.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>