l2tp: fix missing refcount drop in pppol2tp_tunnel_ioctl()

If 'session' is not NULL and is not a PPP pseudo-wire, then we fail to
drop the reference taken by l2tp_session_get().

Fixes: ecd012e4 ("l2tp: filter out non-PPP sessions in pppol2tp_tunnel_ioctl()")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>