xentrace: correct overflow check for number of per-cpu trace pages

xentrace: correct overflow check for number of per-cpu trace pages

The calculated number of per-cpu trace pages is stored in t_info and
shared with tools like xentrace. Since its an u16 the value may
overflow because the current check is based on u32.  Using the u16
means each cpu could in theory use up to 256MB as trace
buffer. However such a large allocation will currently fail on x86 due
to the MAX_ORDER limit.  Check both max theoretical number of pages
per cpu and max number of pages reachable by struct t_buf->prod/cons
variables with requested number of pages.

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