l2tp: ppp: use max L2TP header size for PPP channel hdrlen

l2tp: ppp: use max L2TP header size for PPP channel hdrlen

chan.hdrlen is read once at channel registration by
ppp_register_net_channel(), and used to set the PPP net device's
hard_header_len. It was set to PPPOL2TP_L2TP_HDR_SIZE_NOSEQ (6), which
is 4 bytes too small if sequence numbers are later enabled via
setsockopt(PPPOL2TP_SO_SENDSEQ), causing unnecessary skb reallocations
on the TX path.

The setsockopt handler attempted to change netdev's hard_header_len by
updating chan.hdrlen, but the PPP layer never re-reads it after the
registration, so the update had no effect.

To avoid the unnecessary reallocations, set chan.hdrlen to
PPPOL2TP_L2TP_HDR_SIZE_SEQ (10) unconditionally at registration and
remove the ineffective update in the setsockopt callback.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://patch.msgid.link/20260317054141.524879-1-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>