tcp_splice: Simplify / correct OUT_WAIT flag handling

We set the OUT_WAIT flag if we stop forwarding due to EAGAIN, but there's
still data in the pipe.  That ensures we wake up when the output socket has
room to drain the pipe into.

We clear the OUT_WAIT flag when we complete forwarding on an EPOLLOUT
event, but that's not quite right.  Even though it's called on an EPOLLOUT,
tcp_splice_forward() could, in principle empty the pipe, but also read
enough new data from the other side to fill it again.  That would set
OUT_WAIT internally, but it would be cleared after returning meaning
we could miss a necessary wakeup.

The condition on whether we need write side wakeups is actually fairly
simple: we need them if and only if we return to the main loop with data
in the pipe.  Maintain that in a single place - right after we exit the
forwarding loop in tcp_splice_forward().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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