pppol2tp: Add missing sock_put() in pppol2tp_release()

pppol2tp_sock_to_session() do sock_hold() if the session to release is
not NULL.

Signed-off-by: Frédéric Moulins <frederic.moulins@alsatis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>