Prevent one CPU from getting chosen by concurrent forks

One CPU could be chosen by concurrent forks because CPU selection and
runq addition are not done atomicly. So this fix makes the two steps
atomic.

Change-Id: Ib6b75ad655789385d13207e0a47fa4717dec854a

kernel/include/cls.h

@@ -74,6 +74,7 @@ struct cpu_local_var {
 	struct thread *current;
 	struct list_head runq;
 	size_t runq_len;
+	size_t runq_reserved; /* Number of threads which are about to be added to runq */
 
 	struct ihk_ikc_channel_desc *ikc2linux;