sctp: full support for ipv6 ip_nonlocal_bind & IP_FREEBIND

Commit 9b97420228 ("sctp: support ipv6 nonlocal bind")
introduced support for the above options as v4 sctp did,
so patched sctp_v6_available().

In the v4 implementation it's enough, because
sctp_inet_bind_verify() just returns with sctp_v4_available().
However sctp_inet6_bind_verify() has an extra check before that
for link-local scope_id, which won't respect the above options.

Added the checks before calling ipv6_chk_addr(), but
not before the validation of scope_id.

before (w/ both options):
 ./v6test fe80::10 sctp
 bind failed, errno: 99 (Cannot assign requested address)
 ./v6test fe80::10 tcp
 bind success, errno: 0 (Success)

after (w/ both options):
 ./v6test fe80::10 sctp
 bind success, errno: 0 (Success)

Signed-off-by: Laszlo Toth <laszlth@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/ipv6.c

@@ -882,8 +882,10 @@ static int sctp_inet6_bind_verify(struct sctp_sock *opt, union sctp_addr *addr)
 			net = sock_net(&opt->inet.sk);
 			rcu_read_lock();
 			dev = dev_get_by_index_rcu(net, addr->v6.sin6_scope_id);
-			if (!dev ||
-			    !ipv6_chk_addr(net, &addr->v6.sin6_addr, dev, 0)) {
+			if (!dev || !(opt->inet.freebind ||
+				      net->ipv6.sysctl.ip_nonlocal_bind ||
+				      ipv6_chk_addr(net, &addr->v6.sin6_addr,
+						    dev, 0))) {
 				rcu_read_unlock();
 				return 0;
 			}