ssh: use defaultkey sentinel for ML-DSA-44 default host key

servconf_add_hostkey() compares its file argument to defaultkey by
pointer to skip an unreadable default key; the ML-DSA-44 entry passed
the "[default]" literal, so the skip (and the "No host key files found"
fallback) never applied to it.

MFC after:	1 week
Reviewed by:	emaste
Fixes: bb5c77e9d2 ("OpenSSH: Update to 10.4p1")
Differential Revision:	https://reviews.freebsd.org/D60922

crypto/openssh/servconf.c

@@ -325,7 +325,7 @@ fill_default_server_options(ServerOptions *options)
 #endif
 		servconf_add_hostkey(defaultkey, 0, options,
 		    _PATH_HOST_ED25519_KEY_FILE, 0);
-		servconf_add_hostkey("[default]", 0, options,
+		servconf_add_hostkey(defaultkey, 0, options,
 		    _PATH_HOST_MLDSA44_ED25519_KEY_FILE, 0);
 	}
 	if (options->num_host_key_files == 0)