sched: Add assert to capture rescheduling attempt in nested exception

Rescheduling in nested exception is not supported since nested exception
stack is per cpu, but without the assert such reschedule will cause
stack corruption which will be hard to debug.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>