netrom: info leak in ->getname()

The sockaddr_ax25 struct has a 3 byte hole between ->sax25_call and
->sax25_ndigis.  I've added a memset to avoid leaking uninitialized
stack data to userspace.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>