drivers/net/wan/hdlc_fr: Correctly handle special skb->protocol values

drivers/net/wan/hdlc_fr: Correctly handle special skb->protocol values

The fr_hard_header function is used to prepend the header to skbs before
transmission. It is used in 3 situations:
1) When a control packet is generated internally in this driver;
2) When a user sends an skb on an Ethernet-emulating PVC device;
3) When a user sends an skb on a normal PVC device.

These 3 situations need to be handled differently by fr_hard_header.
Different headers should be prepended to the skb in different situations.

Currently fr_hard_header distinguishes these 3 situations using
skb->protocol. For situation 1 and 2, a special skb->protocol value
will be assigned before calling fr_hard_header, so that it can recognize
these 2 situations. All skb->protocol values other than these special ones
are treated by fr_hard_header as situation 3.

However, it is possible that in situation 3, the user sends an skb with
one of the special skb->protocol values. In this case, fr_hard_header
would incorrectly treat it as situation 1 or 2.

This patch tries to solve this issue by using skb->dev instead of
skb->protocol to distinguish between these 3 situations. For situation
1, skb->dev would be NULL; for situation 2, skb->dev->type would be
ARPHRD_ETHER; and for situation 3, skb->dev->type would be ARPHRD_DLCI.

This way fr_hard_header would be able to distinguish these 3 situations
correctly regardless what skb->protocol value the user tries to use in
situation 3.

Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>