There is already a guard in tcp_output_segment() for a pbuf still being
referenced by the netif driver due to deferred transmission, however the callers
are modifying state even when this gives up.
It seems cleaner to have the callers guard this case and avoid modifying their
state.
tcp_rexmit_rto() might better avoid re-transmission of any segments if any of
the unacked segments are deferred, to avoid loading the link further if it is
struggling to flush its buffered writes. Link level queues can be limited on
some devices and need spares for link management.
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