Fix weird uses of num_online_cpus().

This may be OK in archs with contiguous CPU numbers and without
hotplug CPUs, but it sets a terrible example.

And open-coding it like drivers/scsi/hpsa.c is just weird.

BTRFS has a weird comparison with num_online_cpus() too, but since
BTRFS just screwed up my test machines' root partition, I'm not
touching it :)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reported-by: Oleg Drokin <green@linuxhacker.ru>