wgengine/magicsock: reset endpoint trust on rekey (#20368)

Invalidate the trust window so we re-run path discovery
instead of coasting on the dead path until trust lapses on its own.

Partially reverts 85bb5f8 removal of resetLocked within updateFromNode.

Fixes #20268

Change-Id: Ib18b31e4329513294b682e698d1a9f426a6a6964
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>