l2tp: clean up stale tunnel or session in pppol2tp_connect's error path

pppol2tp_connect() may create a tunnel or a session. Remove them in
case of error.

Fixes: fd558d18 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>