drivers/net/wan/hdlc_x25: Added needed_headroom and a skb->len check

1. Added a skb->len check

This driver expects upper layers to include a pseudo header of 1 byte
when passing down a skb for transmission. This driver will read this
1-byte header. This patch added a skb->len check before reading the
header to make sure the header exists.

2. Added needed_headroom and set hard_header_len to 0

When this driver transmits data,
  first this driver will remove a pseudo header of 1 byte,
  then the lapb module will prepend the LAPB header of 2 or 3 bytes.
So the value of needed_headroom in this driver should be 3 - 1.

Because this driver has no header_ops, according to the logic of
af_packet.c, the value of hard_header_len should be 0.

Reason of setting needed_headroom and hard_header_len at this place:

This driver is written using the API of the hdlc module, the hdlc
module enables this driver (the protocol driver) to run on any hardware
that has a driver (the hardware driver) written using the API of the
hdlc module.

Two other hdlc protocol drivers - hdlc_ppp and hdlc_raw_eth, also set
things like hard_header_len at this place. In hdlc_ppp, it sets
hard_header_len after attach_hdlc_protocol and before setting dev->type.
In hdlc_raw_eth, it sets hard_header_len by calling ether_setup after
attach_hdlc_protocol and after memcpy the settings.

3. Reset needed_headroom when detaching protocols (in hdlc.c)

When detaching a protocol from a hardware device, the hdlc module will
reset various parameters of the device (including hard_header_len) to
the default values. We add needed_headroom here so that needed_headroom
will also be reset.

Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Martin Schiller <ms@dev.tdt.de>
Cc: Andrew Hendry <andrew.hendry@gmail.com>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>