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>
diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 548af48..1ada967 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -1780,6 +1780,7 @@ static int fib6_walk_continue(struct fib6_walker *w)
 			}
 			w->state = FWS_L;
 #endif
+			/* fall through */
 		case FWS_L:
 			left = rcu_dereference_protected(fn->left, 1);
 			if (left) {
@@ -1788,6 +1789,7 @@ static int fib6_walk_continue(struct fib6_walker *w)
 				continue;
 			}
 			w->state = FWS_R;
+			/* fall through */
 		case FWS_R:
 			right = rcu_dereference_protected(fn->right, 1);
 			if (right) {
@@ -1797,6 +1799,7 @@ static int fib6_walk_continue(struct fib6_walker *w)
 			}
 			w->state = FWS_C;
 			w->leaf = rcu_dereference_protected(fn->leaf, 1);
+			/* fall through */
 		case FWS_C:
 			if (w->leaf && fn->fn_flags & RTN_RTINFO) {
 				int err;
@@ -1815,6 +1818,7 @@ static int fib6_walk_continue(struct fib6_walker *w)
 			}
 skip:
 			w->state = FWS_U;
+			/* fall through */
 		case FWS_U:
 			if (fn == w->root)
 				return 0;