Prevent potential concurrent map access by taking a lock before reading the
topology cache's hintsPopulatedByService map.
* staging/src/k8s.io/endpointslice/topologycache/topologycache.go
(setHintsLocked, hasPopulatedHintsLocked): New helper functions. These are
the same as the existing SetHints and HasPopulatedHints methods except that
these helpers assume that a lock is already held.
(SetHints): Use setHintsLocked.
(HasPopulatedHints): Take a lock and use hasPopulatedHintsLocked.
(AddHints): Take a lock and use setHintsLocked and hasPopulatedHintsLocked.
* staging/src/k8s.io/endpointslice/topologycache/topologycache_test.go
(TestTopologyCacheRace): Add a goroutine that calls HasPopulatedHints.
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