Fix T_INFO_FIRST_OFFSET calculation

This wasn't defined correctly, thus allowing in the
num_online_cpus() == NR_CPUS case to pass a corrupted MFN to
Dom0.

Reported-by: Jan Beulich <jbeulich@xxxxxxxxxx>
Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff -r 39f71ef422bd xen/common/trace.c
--- a/xen/common/trace.c	Tue Jun 29 17:23:53 2010 +0100
+++ b/xen/common/trace.c	Wed Jun 30 15:50:33 2010 +0100
@@ -50,8 +50,11 @@
 static struct t_info *t_info;
 #define T_INFO_PAGES 2  /* Size fixed at 2 pages for now. */
 #define T_INFO_SIZE ((T_INFO_PAGES)*(PAGE_SIZE))
-/* t_info.tbuf_size + list of mfn offsets + 1 to round up / sizeof uint32_t */
-#define T_INFO_FIRST_OFFSET ((sizeof(int16_t) + NR_CPUS * sizeof(int16_t) + 1) / sizeof(uint32_t))
+/* Return the number of elements _type necessary to store at least _x bytes of data
+ * i.e., sizeof(_type) * ans >= _x. */
+#define fit_to_type(_type, _x) (((_x)+sizeof(_type)-1) / sizeof(_type))
+/* t_info.tbuf_size + list of mfn offsets rounded up / sizeof uint32_t */
+#define T_INFO_FIRST_OFFSET fit_to_type( uint32_t, (sizeof(int16_t) + NR_CPUS * sizeof(int16_t)) )
 static DEFINE_PER_CPU_READ_MOSTLY(struct t_buf *, t_bufs);
 static DEFINE_PER_CPU_READ_MOSTLY(unsigned char *, t_data);
 static DEFINE_PER_CPU_READ_MOSTLY(spinlock_t, t_lock);