smpboot: fix cachesize comparison in smp_tune_scheduling()

Jarek Poplawski noted that boot_cpu_data.x86_cache_size is signed int
and can be < 0 too.

In fact we test for it. Except we assigned it to an unsigned value..

Cc: Jarek Poplawski <jarkao2@o2.pl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Andi Kleen <ak@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>