Fix a double-free when an inp transitions to INP_TIMEWAIT state after having been dropped.
This fixes enforces in_pcbdrop() logic in tcp_input():
"in_pcbdrop() is used by TCP to mark an inpcb as unused and avoid future packet
delivery or event notification when a socket remains open but TCP has closed."
PR: 203175
Reported by: Slawa Olhovchenkov, Palle Girgensohn, Urmas Lett, imp
Tested by: Slawa Olhovchenkov
MFC after: 1 week