fixestailscale/corp#40401
From an internal stack trace. We witnessed a slow bus error and
a complete lock up of the ipn bus watcher.
Stack showed a b.mu->t.mu and t.mu->b.mu lock ordering
issue when setting up the HealthTracker's initial state and its
timer simultaneously fires.
Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>
c18f8516c4