atm: Allow MSG_PEEK for atm sockets

Now that the vcc backends do the right thing with respect the receive
queue on registration, allow MSK_PEEK for atm sockets.

This allows a userspace program to inspect the packets and decide what
backend to use to handle them.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: David S. Miller <davem@davemloft.net>