powerpc/rtas: allow rescheduling while changing cpu states

rtas_cpu_state_change_mask() potentially operates on scores of cpus,
so explicitly allow rescheduling in the loop body.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802192926.19277-3-nathanl@linux.ibm.com

arch/powerpc/kernel/rtas.c

@@ -16,6 +16,7 @@
 #include <linux/capability.h>
 #include <linux/delay.h>
 #include <linux/cpu.h>
+#include <linux/sched.h>
 #include <linux/smp.h>
 #include <linux/completion.h>
 #include <linux/cpumask.h>
@@ -898,6 +899,7 @@ static int rtas_cpu_state_change_mask(enum rtas_cpu_state state,
 				cpumask_clear_cpu(cpu, cpus);
 			}
 		}
+		cond_resched();
 	}
 
 	return ret;