x86/mm: Fix loop increment in paging_log_dirty_range()

x86/mm: Fix loop increment in paging_log_dirty_range()

In 23417:53ef1f35a0f8 (the fix for XSA-27 / CVE-2012-5511), the
loop variable gets incremented twice, so the loop only clears every
second page of the bitmap.  This might cause the tools to think that
pages are dirty when they are not.

Reported-by: Steven Noonan <snoonan@amazon.com>
Reported-by: Matt Wilson <msw@amazon.com>
Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Jan Beulich <jbeulich@suse.com>