This names the func() that Once-unlocked LocalBackend.mu. It does so
both for docs and because it can then have a method: Unlock, for the
few points that need to explicitly unlock early (the cause of all this
mess). This makes those ugly points easy to find, and also can then
make them stricter, panicking if the mutex is already unlocked. So a
normal call to the func just once-releases the mutex, returning false
if it's already done, but the Unlock method is the strict one.
Then this uses it more, so most the b.mu.Unlock calls remaining are
simple cases and usually defers.
Updates #11649
Change-Id: Ia070db66c54a55e59d2f76fdc26316abf0dd4627
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
aa084a29c6