A subtle oversight would leak information, if a
local session is in the process of being shut
down, as that TCP state transition would be
observable well ahead of the actual transmission
of the FIN.
This would also subtly change new data packet
markings and thereby possibly the treatment of
those by the network (loss rather than marking).
This was found while investigating PR 254725, but
is unrelated to the issue there.
MFC after: 3 days