mptcp: re-arm retransmit timer if data is pending

The retransmit head will be NULL in case there is no in-flight data
(meaning all data injected into network has been acked).

In that case the retransmit timer is stopped.

This is only correct if there is no more pending, not-yet-sent data.

If there is, the retransmit timer needs to set the PENDING bit again so
that mptcp tries to send the remaining (new) data once a subflow can accept
more data.

Also, mptcp_subflow_get_retrans() has to be called unconditionally.

This function checks for subflows that have become unresponsive and marks
them as stale, so in the case where the rtx queue is empty, subflows
will never be marked stale which prevents available backup subflows from
becoming eligible for transmit.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/226


Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>