cpumask: remove __first_cpu / __next_cpu

They were for use by the deprecated first_cpu() and next_cpu() wrappers,
but sparc used them directly.

They're now replaced by cpumask_first / cpumask_next.  And __next_cpu_nr
is completely obsolete.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: David S. Miller <davem@davemloft.net>