cpusets: replace zone allowed functions with node allowed

cpusets: replace zone allowed functions with node allowed

The cpuset_zone_allowed() variants are actually only a function of the
zone's node.

Cc: Paul Menage <menage@google.com>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>