l2tp: don't export tunnel and session free functions

Tunnel and session instances are reference counted, and shouldn't be
directly freed by pseudowire code.

Rather than exporting l2tp_tunnel_free and l2tp_session_free, make them
private to l2tp_core.c, and export the refcount functions instead.

In order to do this, the refcount functions cannot be declared as
inline.  Since the codepaths which take and drop tunnel and session
references are not directly in the datapath this shouldn't cause
performance issues.

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