tcp: Fix upper half (socket code) and so_port switching race.

tcp: Fix upper half (socket code) and so_port switching race.

Use message put done receipt to change this socket's so_port, i.e. _after_
this message was put onto the target netisr's msgport but _before_ the
message could be pulled from the target netisr's msgport, so that:
- The upper half (socket code) will not see the new msgport before this
  message reaches the new msgport and messages for this socket will be
  ordered.
- This message will see the new msgport, when its handler is called in the
  target netisr.

Add comment about it and explain why the so_port switching should be done
in this commit's fashion.

Reported-by: dillon@