l2tp: define parameters of l2tp_session_get*() as "const"

Make l2tp_pernet()'s parameter constant, so that l2tp_session_get*() can
declare their "net" variable as "const".
Also constify "ifname" in l2tp_session_get_by_ifname().

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