net: ax25: fix information leak to userland harder

Commit fe10ae53 adds a memset() to clear
the structure being sent back to userspace, but accidentally used the
wrong size.

Reported-by: Brad Spengler <spender@grsecurity.net>
Signed-off-by: Kees Cook <kees.cook@canonical.com>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>