sched/sched: SMP: Fix this_task() to be an atomic operation. In the previous...

sched/sched:  SMP: Fix this_task() to be an atomic operation.  In the previous implementation, this_task() was defined in sched.h by using just a macro current_task(this_cpu()).  However, I found that this is not atomic and actually sometimes switching CPU happened in executing the macro when we tested audio steaming plus executing commands via telnet.  This change resolves this issue by implementing atomic this_task()in sched_thistask.c which is newly introduced.