l2tp: simplify tunnel and socket cleanup

When the l2tp tunnel socket used sk_user_data to point to its
associated l2tp tunnel, socket and tunnel cleanup had to make use of
the socket's destructor to free the tunnel only when the socket could
no longer be accessed.

Now that sk_user_data is no longer used, we can simplify socket and
tunnel cleanup:

  * If the tunnel closes first, it cleans up and drops its socket ref
    when the tunnel refcount drops to zero. If its socket was provided
    by userspace, the socket is closed and freed asynchronously, when
    userspace closes it. If its socket is a kernel socket, the tunnel
    closes the socket itself during cleanup and drops its socket ref
    when the tunnel's refcount drops to zero.

  * If the socket closes first, we initiate the closing of its
    associated tunnel. For UDP sockets, this is via the socket's
    encap_destroy hook. For L2TPIP sockets, this is via the socket's
    destroy callback. The tunnel holds a socket ref while it
    references the sock. When the tunnel is freed, it drops its socket
    ref and the socket will be cleaned up when its own refcount drops
    to zero, asynchronous to the tunnel free.

  * The tunnel socket destructor is no longer needed since the tunnel
    is no longer freed through the socket destructor.

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>