net: context: let tcp_established() handle more TCP states

Due to commit fece8569 ("net: tcp: Clean up FIN handling") the
tcp_established() callback now handles TCP connections which are
in various ending/closing states other than TCP_ESTABLISHED.

Currently, these states are generating the following error and not
being processed:
Context 0x123456778 in wrong state 6.
(Shown when TCP is in LAST_ACK state).

This commit also fixes a memory leak issue discribed in
Jira: ZEP-1658

Analysis of the memory leak issue is here:
When TCP connection is established, tcp context is in
NET_TCP_ESTABLISHED state. Once it receives FIN message from client
it goes to NET_TCP_CLOSE_WAIT and then it turns to NET_TCP_LAST_ACK
after connection closing request from server. Now server gets final
ack from client, but tcp_established() will reject it because current
state is not in NET_TCP_ESTABLISHED. Even if server receives proper
ack, it is not handled by server. Hence 'sent_list' is not freed.

Change-Id: I41c8af2e6851809f87a02c271a4290cf3d823ebb
Signed-off-by: Michael Scott <michael.scott@linaro.org>