xentrace: reduce trace buffer size to something mfn_offset can reach

xentrace: reduce trace buffer size to something mfn_offset can reach

The start of the array which holds the list of mfns for each cpus
tracebuffer is stored in an unsigned short. This limits the total
amount of pages for each cpu as the number of active cpus increases.

Update the math in calculate_tbuf_size() to apply also this rule to
the max number of trace pages. Without this change the index can
overflow.

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