livedump: Silence KASAN and KMSAN when livedumping

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.

Reviewed by: mhorne, khng
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47714

(cherry picked from commit 37cef00192ef1347d27ac64be419267a4ba604ac)