net: l2: ppp: Allow PPP to transtition ESTABLISH->DEAD

[SeppoTakalo]

When remote peer have closed the PPP link normally (LCP Terminate-Request), the PPP stack on Zephyr side switches back to ESTABLISH phase to be ready for next handshake.

When calling net_if_down() on the interface, it should not try to initiate LCP link termination, but instead go directly to DEAD phase.

See https://datatracker.ietf.org/doc/html/rfc1661#section-3.2

   +------+        +-----------+           +--------------+
   |      | UP     |           | OPENED    |              | SUCCESS/NONE
   | Dead |------->| Establish |---------->| Authenticate |--+
   |      |        |           |           |              |  |
   +------+        +-----------+           +--------------+  |
      ^               |                        |             |
      |          FAIL |                   FAIL |             |
      +<--------------+             +----------+             |
      |                             |                        |
      |            +-----------+    |           +---------+  |
      |       DOWN |           |    |   CLOSING |         |  |
      +------------| Terminate |<---+<----------| Network |<-+
                   |           |                |         |
                   +-----------+                +---------+

ESTABLISH->DEAD is a valid transition.
It does not have to go to ESTABLISH->TERMINATE->DEAD.

Without this change, the net_if_down() waits for CONFIG_NET_L2_PPP_TIMEOUT until returning if the link was already closed by the remote peer. If the link was up, the termination is faster as peer is responding.

This is because when reaching the DEAD phase, the lcp_down() returns to ESTABLISH if we still indicate carrier is OK.
https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/net/l2/ppp/lcp.c#L208-L210

This delay basically only affects the peer which is serving the connection (PPP server/modem).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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