xentrace: fix off-by-one in calculate_tbuf_size

Commit "xentrace: reduce trace buffer size to something mfn_offset can
reach" contains an off-by-one bug. max_mfn_offset needs to be reduced by
exactly the value of t_info_first_offset.

If the system has two cpus and the number of requested trace pages is
very large, the final number of trace pages + the offset will not fit
into a short. As a result the variable offset in alloc_trace_bufs() will
wrap while allocating buffers for the second cpu. Later
share_xen_page_with_privileged_guests() will be called with a wrong page
and the ASSERT in this function triggers. If the ASSERT is ignored by
running a non-dbg hypervisor the asserts in xentrace itself trigger
because "cons" is not aligned because the very last trace page for the
second cpu is a random mfn.

Thanks to Jan for the quick analysis.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>

xen/common/trace.c

@@ -133,7 +133,7 @@ static int calculate_tbuf_size(unsigned int pages, uint16_t t_info_first_offset)
      * The array of mfns for the highest cpu can start at the maximum value
      * mfn_offset can hold. So reduce the number of cpus and also the mfn_offset.
      */
-    max_mfn_offset -= t_info_first_offset - 1;
+    max_mfn_offset -= t_info_first_offset;
     max_cpus--;
     if ( max_cpus )
         max_mfn_offset /= max_cpus;