sched: use alloc_cpumask_var() instead of alloc_bootmem_cpumask_var()

Slab is initialized when sched_init() runs now so lets use alloc_cpumask_var().

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>