When attempting to transmit a packet, if the system fails to allocate a mbuf, it may return without setting any timers.  If no more data is scheduled to be transmitted (this was a FIN) the system will sit in LAST_ACK state forever.
When attempting to transmit a packet, if the system fails to allocate
a mbuf, it may return without setting any timers.  If no more data is
scheduled to be transmitted (this was a FIN) the system will sit in
LAST_ACK state forever.

Thus, when mbuf allocation fails, set the retransmit timer if neither
the retransmit or persist timer is already pending.

Problem discovered by:  Mike Silbersack (silby@silby.com)
Pushed for a fix by:    Bosko Milekic <bmilekic@dsuper.net>
Reviewed by:            jayanth

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