ppp: don't store tx skb in the fastpath

ppp: don't store tx skb in the fastpath

Currently, ppp->xmit_pending is used in ppp_send_frame() to pass a skb
to ppp_push(), and holds the skb when a PPP channel cannot immediately
transmit it. This state is redundant because the transmit queue
(ppp->file.xq) can already handle the backlog. Furthermore, during
normal operation, an skb is queued in file.xq only to be immediately
dequeued, causing unnecessary overhead.

Refactor the transmit path to avoid stashing the skb when possible:
- Remove ppp->xmit_pending.
- Rename ppp_send_frame() to ppp_prepare_tx_skb(), and don't call
  ppp_push() in it. It returns 1 if the skb is consumed
  (dropped/handled) or 0 if it can be passed to ppp_push().
- Update ppp_push() to accept the skb. It returns 1 if the skb is
  consumed, or 0 if the channel is busy.
- Optimize __ppp_xmit_process():
  - Fastpath: If the queue is empty, attempt to send the skb directly
    via ppp_push(). If busy, queue it.
  - Slowpath: If the queue is not empty, process the backlog in
    file.xq. Split dequeuing loop into a separate function
    ppp_xmit_flush() so ppp_channel_push() uses that directly instead of
    passing a NULL skb to __ppp_xmit_process().

This simplifies the states and reduces locking in the fastpath.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20260303093219.234403-1-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>