ipv6: fib: fix fib dump restart

Commit 2bec5a36 (ipv6: fib: fix crash when changing large fib
while dumping it) introduced ability to restart the dump at tree root,
but failed to skip correctly a count of already dumped entries. Code
didn't match Patrick intent.

We must skip exactly the number of already dumped entries.

Note that like other /proc/net files or netlink producers, we could
still dump some duplicates entries.
Reported-by: Debabrata Banerjee <dbavatar@gmail.com>
Reported-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

(cherry picked from commit fa809e2f)
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

net/ipv6/ip6_fib.c

@@ -1420,7 +1420,7 @@ static int fib6_walk_continue(struct fib6_walker_t *w)
 
 				if (w->skip) {
 					w->skip--;
-					continue;
+					goto skip;
 				}
 
 				err = w->func(w);
@@ -1430,6 +1430,7 @@ static int fib6_walk_continue(struct fib6_walker_t *w)
 				w->count++;
 				continue;
 			}
+skip:
 			w->state = FWS_U;
 		case FWS_U:
 			if (fn == w->root)