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>

arch/mips/include/asm/mach-ip27/topology.h

@@ -37,7 +37,6 @@ extern unsigned char __node_distances[MAX_COMPACT_NODES][MAX_COMPACT_NODES];
 
 /* sched_domains SD_NODE_INIT for SGI IP27 machines */
 #define SD_NODE_INIT (struct sched_domain) {		\
-	.span			= CPU_MASK_NONE,	\
 	.parent			= NULL,			\
 	.child			= NULL,			\
 	.groups			= NULL,			\