[NET]: Disable NET_SCH_CLK_CPU for SMP x86 hosts

Opterons with frequency scaling have fully unsynchronized TSCs
running at different frequencies, so using TSCs there is not a good idea.
Also some other x86 boxes have this problem. gettimeofday should be good
enough, so just disable it.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>