Fix SMP race in NMI watchdog on i386/x86-64

Fix a long standing SMP Setup race in the NMI watchdog.  The watchdog would
tick from very early and check if all CPUs increase their timer interrupts.
For that it would check the cpu_online_map.  Now if a CPU took too long to
boot the watchdog would trigger prematurely because the CPU didn't increase
its timer count yet.

Fix is to check cpu_callin_map instead of cpu_online_map because the first is
only set when a CPU started its timer interrupt.

I fixed it on i386 and x86-64.

Description of the problem from Manpreet Singh. Thanks.

Cc: <manpreet@fabric7.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>