Bluetooth: Fix leak of uninitialized data to userspace

    struct hci_dev_list_req {
            __u16  dev_num;
            struct hci_dev_req dev_req[0];  /* hci_dev_req structures */
    };

sizeof(struct hci_dev_list_req) == 4, so the two bytes immediately
following "dev_num" will never be initialized. When this structure
is copied to userspace, these uninitialized bytes are leaked.

Fix by using kzalloc() instead of kmalloc(). Found using kmemcheck.

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>