ppp: disconnect channel before nullifying pch->chan

ppp: disconnect channel before nullifying pch->chan

In ppp_unregister_channel(), pch->chan is set to NULL before calling
ppp_disconnect_channel(), which removes the channel from ppp->channels
list using list_del_rcu() + synchronize_net(). This creates an
intermediate state where the channel is still connected (on the list)
but already unregistered (pch->chan == NULL).

Call ppp_disconnect_channel() before setting pch->chan to NULL. After
the synchronize_net(), no new reader on the transmit path will hold a
reference to the channel from the list.

This eliminates the problematic state, and prepares for removing the
pch->chan NULL checks from the transmit path in a subsequent patch.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20260312093732.277254-1-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>