The mayday trap asks the co-kernel to deal with a runaway context
which has been detected earlier. When doing so, the co-kernel might
re-enable hard interrupts, which we have to disable again before
unwinding the IRQ context which triggered the whole process (like a
watchdog event).

Use __ipipe_call_mayday() to fire the notification to the co-kernel
instead of the open coded version, so that we do restore the hard
interrupt state properly before leaving __ipipe_exit_irq().