l2tp: cleanup eth/ppp pseudowire setup code

l2tp eth/ppp pseudowire setup/cleanup uses kfree() in some error
paths. Drop the refcount instead such that the session object is
always freed when the refcount reaches 0.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>