[IrDA] af_irda: Silence kernel message in irda_recvmsg_stream

This patch silences an IRDA_ASSERT in irda_recvmsg_stream, as described in
http://bugzilla.kernel.org/show_bug.cgi?id=7512

 irda_disconnect_indication
would set sk->sk_err to ECONNRESET, and a subsequent call to recvmsg
would print an irritating kernel message and return -1.

When a connected socket is closed by the peer, recvmsg should return 0
rather than an error. This patch fixes this.

Signed-off-by: Olaf Kirch <olaf.kirch@oracle.com>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>