BUG/MEDIUM: stconn: Don't forward shutdown to SE if iobuf is not empty

BUG/MEDIUM: stconn: Don't forward shutdown to SE if iobuf is not empty

It is only an issue when the kernel splicing is used. The zero-copy
forwarding via the buffers is not affected. When a shutdown is received on
the producer side and some data are blocked in the pipe for a while, the
shutdown may be forwarded to the other side. Usually, in this case, the
shutdown must be scheduled, waiting all output data (from the channel and
the consumer's iobuf) are sent. But only the channel was considered.

The bug was introduced by commit 20c463955d ("MEDIUM: channel: don't look at
iobuf to report an empty channel"). To fix the issue, we must also check
data blocked in the consummer iobuf.

This patch should solve the issue #2505. It must be backported to 2.9.