atm: sk_wmem_alloc initial value is one

commit 2b85a34e
(net: No more expensive sock_hold()/sock_put() on each tx)
changed initial sk_wmem_alloc value.

This broke net/atm since this protocol assumed a null
initial value. This patch makes necessary changes.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>