compat/gnttab: Prevent infinite loop in compat code

compat/gnttab: Prevent infinite loop in compat code

c/s 20281:95ea2052b41b, which introduces Grant Table version 2
hypercalls introduces a vulnerability whereby the compat hypercall
handler can fall into an infinite loop.

If the watchdog is enabled, Xen will die after the timeout.

This is a security problem, XSA-24 / CVE-2012-4539.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>