pppoatm: drop frames to not-ready vcc

pppoatm: drop frames to not-ready vcc

The vcc_destroy_socket() closes vcc before the protocol is detached
from vcc by calling vcc->push() with NULL skb. This leaves some time
window, where the protocol may call vcc->send() on closed vcc
and crash.

Now pppoatm_send(), like vcc_sendmsg(), checks for vcc flags that
indicate that vcc is not ready. If the vcc is not ready we just
drop frame. Queueing frames is much more complicated because we
don't have callbacks that inform us about vcc flags changes.

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