libndp: validate the IPv6 hop limit

None of the NDP messages should ever come from a non-local network; as
stated in RFC4861's 6.1.1 (RS), 6.1.2 (RA), 7.1.1 (NS), 7.1.2 (NA),
and 8.1. (redirect):

  - The IP Hop Limit field has a value of 255, i.e., the packet
    could not possibly have been forwarded by a router.

This fixes CVE-2016-3698.

Reported by: Julien BERNARD <julien.bernard@viagenie.ca>
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
static void log_stderr(struct ndp *ndp, int priority,
		       const char *file, int line, const char *fn,
		       const char *format, va_list args)
{
	fprintf(stderr, "libndp: %s: ", fn);
	vfprintf(stderr, format, args);
	fprintf(stderr, "\n");
}

static void log_stderr(struct ndp *ndp, int priority,
		       const char *file, int line, const char *fn,
		       const char *format, va_list args)
{
	fprintf(stderr, "libndp: %s: ", fn);
	vfprintf(stderr, format, args);
	fprintf(stderr, "\n");
}
