The bpq ether driver is modifying the data art of the skb by first
dropping the KISS byte (a command byte for the radio) then prepending the
length + 4 of the remaining AX.25 packet to be transmitted as a little
endian 16-bit number.  If the high byte of the length has a different
value than the dropped KISS byte users of clones of the skb may observe
this as corruption.  This was observed with by running listen(8) -a which
uses a packet socket which clones transmit packets.  The corruption will
then typically be displayed for as a KISS "TX Delay" command for AX.25
packets in the range of 252..508 bytes or any other KISS command for
yet larger packets.

Fixed by using skb_cow to create a private copy should the skb be cloned.
Using skb_cow also allows us to cleanup the old logic to ensure sufficient
headroom in the skb.

While at it, replace a return of 0 from bpq_xmit with the proper constant
NETDEV_TX_OK which is now being used everywhere else in this function.

Affected: all 2.2, 2.4 and 2.6 kernels.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Reported-by: Jann Traschewski <jann@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>