cmd/derper: fix regression from bootstrap DNS optimization

The commit b9c92b90db earlier today
caused a regression of serving an empty map always, as it was
JSON marshalling an atomic.Value instead of the DNS entries map
it just built.

Change-Id: I9da3eeca132c6324462dedeaa7d002908557384b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f