xfrm: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: KSPP#115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Author: GustavoARSilva

net/xfrm/xfrm_interface.c

@@ -432,6 +432,7 @@ static int xfrmi4_err(struct sk_buff *skb, u32 info)
 	case ICMP_DEST_UNREACH:
 		if (icmp_hdr(skb)->code != ICMP_FRAG_NEEDED)
 			return 0;
+		break;
 	case ICMP_REDIRECT:
 		break;
 	default: