cpumask: Introduce cpumask_of_{node,pcibus} to replace {node,pcibus}_to_cpumask

Impact: New APIs

The old node_to_cpumask/node_to_pcibus returned a cpumask_t: these
return a pointer to a struct cpumask.  Part of removing cpumasks from
the stack.

This defines them in the generic non-NUMA case.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Richard Henderson <rth@twiddle.net>