cpumask: convert node_to_cpumask_map[] to cpumask_var_t

Impact: fix (CONFIG_MAXSMP=y only) boot crash

c032ef60

 "cpumask: convert
node_to_cpumask_map[] to cpumask_var_t" didn't get this one
conversion.  There was a compile warning, but I missed it.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Mike Travis <travis@sgi.com>
LKML-Reference: <200903132342.42813.rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>