xen/trace: Fix trace metadata page count calculation (revert fbf96e6)

Changeset fbf96e6, "xentrace: correct formula to calculate
t_info_pages", broke the trace metadata page count calculation, by
mistaking t_info_first_offset as denominated in bytes, when in fact it
is denominated in words (uint32_t).

Effectively revert that change, and put a comment there to reduce the
chance that someone will make that mistake in the future.

Reviewed-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Tested-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>

xen/common/trace.c

@@ -148,8 +148,12 @@ static int calculate_tbuf_size(unsigned int pages, uint16_t t_info_first_offset)
         pages = max_pages;
     }
 
-    t_info_words = num_online_cpus() * pages * sizeof(uint32_t);
-    t_info_pages = PFN_UP(t_info_first_offset + t_info_words);
+    /* 
+     * NB this calculation is correct, because t_info_first_offset is
+     * in words, not bytes, not bytes
+     */
+    t_info_words = num_online_cpus() * pages + t_info_first_offset;
+    t_info_pages = PFN_UP(t_info_words * sizeof(uint32_t));
     printk(XENLOG_INFO "xentrace: requesting %u t_info pages "
            "for %u trace pages on %u cpus\n",
            t_info_pages, pages, num_online_cpus());