cpumask: Use cpu_*_mask accessors code: alpha

Impact: use new API, fix SMP bug.

Use the new accessors rather than frobbing bits directly.

This also removes the bug introduced in ee0c468b

 (alpha: compile
fixes) which had Alpha setting bits on an on-stack cpumask, not the
cpu_online_map.

Cc: Richard Henderson <rth@twiddle.net>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Mike Travis <travis@sgi.com>
Acked-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Acked-by: Ingo Molnar <mingo@elte.hu>