[XFRM]: beet: use IPOPT_NOP for option padding

draft-nikander-esp-beet-mode-07.txt states "The padding MUST be filled
with NOP options as defined in Internet Protocol [1] section 3.1
Internet header format.", so do that.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/xfrm4_mode_beet.c

@@ -54,6 +54,8 @@ static int xfrm4_beet_output(struct xfrm_state *x, struct sk_buff *skb)
 		ph->padlen = 4 - (optlen & 4);
 		ph->hdrlen = (optlen + ph->padlen + sizeof(*ph)) / 8;
 		ph->nexthdr = top_iph->protocol;
+		if (ph->padlen)
+			memset(ph + 1, IPOPT_NOP, ph->padlen);
 
 		top_iph->protocol = IPPROTO_BEETPH;
 		top_iph->ihl = sizeof(struct iphdr) / 4;