sched: convert struct root_domain to cpumask_var_t, fix

Mathieu Desnoyers reported this build failure on powerpc:

 kernel/sched.c: In function 'sd_init_NODE':
 kernel/sched.c:7319: error: non-static initialization of a flexible array member
 kernel/sched.c:7319: error: (near initialization for '(anonymous)')

this happens because .span changed to cpumask_var_t, hence
the static CPU_MASK_NONE initializers in the SD_*_INIT
templates are not type-correct anymore.

Remove them, as they default to empty anyway.

Also remove them from IA64, MIPS and SH.

Reported-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>