l2tp: remove pppol2tp_session_ioctl()

pppol2tp_ioctl() has everything in place for handling PPPIOCGL2TPSTATS
on session sockets. We just need to copy the stats and set ->session_id.

As a side effect of sharing session and tunnel code, ->using_ipsec is
properly set even when the request was made using a session socket.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>