CID 352776: Concurrent data access violations (MISSING_LOCK)

Currently only affects v9.19 because !6250 (merged) is still not backported. @matthijs please update the labels when/if it gets backported.

*** 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);