When querying for an exit node suggestion, occasionally it triggers a
new report concurrently with an existing report in progress. Generally,
there should always be a recent report or one in progress, so it is
redundant to start one there, and it causes concurrency issues.
Fixes#12643
Change-Id: I66ab9003972f673e5d4416f40eccd7c6676272a5
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>