sctp: label accepted/peeled off sockets

Accepted or peeled off sockets were missing a security label (e.g.
SELinux) which means that socket was in "unlabeled" state.

This patch clones the sock's label from the parent sock and resolves the
issue (similar to AF_BLUETOOTH protocol family).

Cc: Paul Moore <pmoore@redhat.com>
Cc: David Teigland <teigland@redhat.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: David S. Miller <davem@davemloft.net>