Lock check of DNS_ZONEFLG_EXITING flag

Lock check of DNS_ZONEFLG_EXITING flag

    WARNING: ThreadSanitizer: data race
    Read of size 4 at 0x000000000001 by thread T1 (mutexes: write M1):
    #0 dns_zone_refresh lib/dns/zone.c:10447
    #1 dns_zone_notifyreceive2 lib/dns/zone.c:13646
    #2 dns_zone_notifyreceive2 lib/dns/zone.c:13497
    #3 ns_notify_start bin/named/notify.c:150
    #4 client_request bin/named/client.c:3137
    #5 dispatch lib/isc/task.c:1157
    #6 run lib/isc/task.c:1331
    #7 <null> <null>

    Previous write of size 4 at 0x000000000001 by thread T2 (mutexes: write M2):
    #0 dns_zone_refresh lib/dns/zone.c:10466
    #1 zone_maintenance lib/dns/zone.c:10236
    #2 zone_timer lib/dns/zone.c:13136
    #3 dispatch lib/isc/task.c:1157
    #4 run lib/isc/task.c:1331
    #5 <null> <null>