8296615: use of undeclared identifier 'IPV6_DONTFRAG'

Reviewed-by: michaelm

Author: XueleiFan

src/jdk.net/macosx/native/libextnet/MacOSXSocketOptions.c

@@ -38,6 +38,10 @@
 #define IP_DONTFRAG             28
 #endif
 
+#ifndef IPV6_DONTFRAG
+#define IPV6_DONTFRAG           62
+#endif
+
 #include "jni_util.h"
 
 /*