ppp: enable TX scatter-gather

ppp: enable TX scatter-gather

PPP channels using chan->direct_xmit prepend the PPP header to a skb and
call dev_queue_xmit() directly. In this mode the skb does not need to be
linear, but the PPP netdevice currently does not advertise
scatter-gather features, causing unnecessary linearization and
preventing GSO.

Enable NETIF_F_SG and NETIF_F_FRAGLIST on PPP devices. In case a linear
buffer is required (PPP compression, multilink, and channels without
direct_xmit), call skb_linearize() explicitly.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20260129012902.941-1-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>