livedump: Silence KASAN and KMSAN when livedumping

[markj]

Summary

The livedumper triggers reports from both of these sanitizers since it
necessarily accesses uninitialized or freed memory. Add a flag to
silence reports from both sanitizers.

[markj]

Check for curthread == NULL, which can happen while APs are executing
init_secondary().

[mhorne]

Makes sense to me.