sctp: full support for ipv6 ip_nonlocal_bind & IP_FREEBIND

Commit 9b974202 ("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>