Address data race in dns_stats_detach over references

Address data race in dns_stats_detach over references

    WARNING: ThreadSanitizer: data race
    Write of size 4 at 0x000000000001 by thread T1 (mutexes: write M1):
    #0 dns_stats_detach lib/dns/stats.c:115:19
    #1 destroy lib/dns/view.c:527:3
    #2 dns_view_weakdetach lib/dns/view.c:704:3
    #3 zone_free lib/dns/zone.c:1149:3
    #4 zone_shutdown lib/dns/zone.c:13123:3
    #5 dispatch lib/isc/task.c:1157:7
    #6 run lib/isc/task.c:1331:2

    Previous read of size 4 at 0x000000000001 by thread T2:
    #0 dns_stats_detach lib/dns/stats.c:118:13
    #1 destroy lib/dns/view.c:527:3
    #2 dns_view_weakdetach lib/dns/view.c:704:3
    #3 zone_free lib/dns/zone.c:1152:3
    #4 zone_shutdown lib/dns/zone.c:13123:3
    #5 dispatch lib/isc/task.c:1157:7
    #6 run lib/isc/task.c:1331:2