ref: makes correction to SCTPConn close logic for SCTPWrite

[gomaja]

Here is a PR that makes a correction to Close

fd := atomic.SwapInt32(&c._fd, -1) // fd = old value, c._fd = -1
c.SCTPWrite(nil, info) // BROKEN: uses c.fd() which returns -1

[gomaja]

@ishidawataru after the fix, the c.SCTPWrite(nil, info) is returning an error "invalid argument"

I think SCTPWrite can be removed completely from Close.

[gomaja]

This code part "defer" after an error with close is not performing well in dialSCTPExtConfig, specifically when SCTPConnect is returning one of the errors (syscall.EISCONN, syscall.EALREADY, syscall.EINPROGRESS). Mostly syscall.EINPROGRESS.
The close in defer is not completely closing the socket, and the next call of dialSCTPExtConfig is always giving syscall.EADDRINUSE. The issue can be fixed by adding also syscall.Shutdown(sock, syscall.SHUT_RDWR). However, I solved it with a more general close.

defer func() {
		if err != nil {
			syscall.Close(sock)
		}
	}()

[gomaja]

PR ready for review,. Changes have made operation very smooth as previously it was very faulty specifically when the connection sends to a backhole (peer no more reachable) while having many messages to send (Full send-Q).

Later we can refactor syscall.SendmsgN(c.fd(), b, cbuf, nil, syscall.MSG_DONTWAIT) to a better one.