ADB entries could be unlinked too soon

due to a typo in the code, ADB entries were unlinked from their entry
buckets during shutdown if they had a nonzero reference count. they
were only supposed to be unlinked if the reference count was exactly
one (that being the reference held by the bucket itself).
65 jobs for !6009 with 3218-enable-the-deferred-fatality-of-missing-glue-servers-from-bind-9-5-0 in 18 minutes and 42 seconds (queued for 3 seconds)
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