Fix CID 352776: Concurrent data access violations

Fix CID 352776: Concurrent data access violations

*** CID 352776:  Concurrent data access violations  (MISSING_LOCK)
/lib/dns/dst_api.c: 474 in dst_key_setmodified()
468     dst_key_isexternal(dst_key_t *key) {
469		return (key->external);
470     }
471
472     void
473     dst_key_setmodified(dst_key_t *key, bool value) {
>>>     CID 352776:  Concurrent data access violations  (MISSING_LOCK)
>>>     Accessing "key->modified" without holding lock
>>>	"dst_key.mdlock". Elsewhere, "dst_key.modified" is accessed with
>>>	"dst_key.mdlock" held 8 out of 11 times (8 of these accesses
>>>	strongly imply that it is necessary).
474		key->modified = value;
475     }
476
477     bool
478     dst_key_ismodified(dst_key_t *key) {
479		return (key->modified);