atm: add release_cb() callback to vcc

The immediate use case for this is that it will allow us to ensure that a
pppoatm queue is woken after it has to drop a packet due to the sock being
locked.

Note that 'release_cb' is called when the socket is *unlocked*. This is
not to be confused with vcc_release() — which probably ought to be called
vcc_close().

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Krzysztof Mazur <krzysiek@podlesie.net>