ppp: remove pch->chan NULL checks from tx path

ppp: remove pch->chan NULL checks from tx path

Now that ppp_disconnect_channel() is called before pch->chan is set to
NULL, a channel from ppp->channels list on the transmit path is
guaranteed to have non-NULL pch->chan.

Remove the pch->chan NULL checks from ppp_push(), ppp_mp_explode(), and
ppp_fill_forward_path(), where a channel is obtained from the list.
Remove the corresponding WRITE/READ_ONCE annotations as they no longer
race.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20260312093732.277254-2-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>