(for 4.9.3) CVE-2018-16229/DCCP: Fix printing "Timestamp" and "Timestamp Echo" options

Add some comments.

Moreover:
Put a function definition name at the beginning of the line.

(This change was ported from commit 6df4852 in the master branch.)

Ryan Ackroyd had independently identified this buffer over-read later by
means of fuzzing and provided the packet capture file for the test.
static const char *dccp_reset_code(uint8_t code)
{
	if (code >= __DCCP_RESET_CODE_LAST)
		return "invalid";
	return dccp_reset_codes[code];
}

static const char *dccp_reset_code(uint8_t code)
{
	if (code >= __DCCP_RESET_CODE_LAST)
		return "invalid";
	return dccp_reset_codes[code];
}
