ipv6: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in some cases I placed the "fall through" comment
on its own line, which is what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ah6.c

diff --git a/net/ipv6/ah6.c b/net/ipv6/ah6.c
index 7802b72..37bb33f 100644
--- a/net/ipv6/ah6.c
+++ b/net/ipv6/ah6.c
@@ -271,6 +271,7 @@ static int ipv6_clear_mutable_options(struct ipv6hdr *iph, int len, int dir)
 		case NEXTHDR_DEST:
 			if (dir == XFRM_POLICY_OUT)
 				ipv6_rearrange_destopt(iph, exthdr.opth);
+			/* fall through */
 		case NEXTHDR_HOP:
 			if (!zero_out_mutable_opts(exthdr.opth)) {
 				net_dbg_ratelimited("overrun %sopts\n",