dir/server: fix vet error in array of locks
dir/server: fix vet error in array of locks

Change-Id: I35107ddcf8c99d8d3ec7875b9861265736e99e52
Reviewed-on: https://upspin-review.googlesource.com/7760
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

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