net: constify some ppp/pptp structs

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/pppoe.c
Simple merge

diff --cc drivers/net/pppox.c
Simple merge

diff --cc drivers/net/pptp.c
Simple merge

diff --cc include/linux/if_pppox.h
Simple merge

diff --cc net/l2tp/l2tp_ppp.c
Simple merge