cpumask: make CONFIG_NR_CPUS always valid.

Impact: cleanup

Currently we have NR_CPUS, which is 1 on UP, and CONFIG_NR_CPUS on
SMP.  If we make CONFIG_NR_CPUS always valid (and always 1 on !SMP),
we can skip the middleman.

This also allows us to find and check all the unaudited NR_CPUS usage
as we prepare for v. large NR_CPUS.

To avoid breaking every arch, we cheat and do this for the moment
in the header if the arch doesn't.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>