[ipv6] Handle IPv6 option length correctly

The IPv6 option length field represents the length of the option data
field, not the overall length of the option.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv6.c

 		if ( option->type == IPV6_OPT_PAD1 ) {
 			option = ( ( ( void * ) option ) + 1 );
 		} else {
-			option = ( ( ( void * ) option ) + option->len );
+			option = ( ( ( void * ) option->value ) + option->len );
 		}
 	}
 	return 0;