ppp: fix race conditions in ppp_fill_forward_path

ppp: fix race conditions in ppp_fill_forward_path

ppp_fill_forward_path() has two race conditions:

1. The ppp->channels list can change between list_empty() and
   list_first_entry(), as ppp_lock() is not held. If the only channel
   is deleted in ppp_disconnect_channel(), list_first_entry() may
   access an empty head or a freed entry, and trigger a panic.

2. pch->chan can be NULL. When ppp_unregister_channel() is called,
   pch->chan is set to NULL before pch is removed from ppp->channels.

Fix these by using a lockless RCU approach:
- Use list_first_or_null_rcu() to safely test and access the first list
  entry.
- Convert list modifications on ppp->channels to their RCU variants and
  add synchronize_net() after removal.
- Check for a NULL pch->chan before dereferencing it.

Fixes: f6efc675c9dd ("net: ppp: resolve forwarding path for bridge pppoe devices")
Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20250814012559.3705-2-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>