sctp: copy the optval from user space in sctp_setsockopt

Prepare for for moving the copy_from_user from the individual sockopts
to the main setsockopt helper.  As of this commit the kopt variable
is not used yet, but the following commits will start using it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>