sctp doesn't honour net.ipv6.bindv6only

The sctp code doesn't use sk->sk_ipv6only (which is initialised
from net.ipv6.bindv6only) but instead uses its own flag
sp->v4mapped which always defaults to 1.

There may also be an expectation that
  [gs]etsockopt(sctp_fd, IPPROTO_IPV6, IPV6_V6ONLY,...)
will access the flag that sctp uses internally.
(Matching TCP and UDP.)

Patch is trivial (remembering the inverted state)
but I don't have a clean enough source tree :-(

	David

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