In places where we are locking the LocakBackend and immediately deferring an
unlock, and where there is no shortcut path in the control flow below the
deferral, we do not need the unlockOnce helper. Replace all these with use of
the lock directly.
Updates #11649
Change-Id: I3e6a7110dfc9ec6c1d38d2585c5367a0d4e76514
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>