ax25: Stop using magic neighbour cache operations.

Before the ax25 stack calls dev_queue_xmit it always calls
ax25_type_trans which sets skb->protocol to ETH_P_AX25.

Which means that by looking at the protocol type it is possible to
detect IP packets that have not been munged by the ax25 stack in
ndo_start_xmit and call a function to munge them.

Rename ax25_neigh_xmit to ax25_ip_xmit and tweak the return type and
value to be appropriate for an ndo_start_xmit function.

Update all of the ax25 devices to test the protocol type for ETH_P_IP
and return ax25_ip_xmit as the first thing they do.  This preserves
the existing semantics of IP packet processing, but the timing will be
a little different as the IP packets now pass through the qdisc layer
before reaching the ax25 ip packet processing.

Remove the now unnecessary ax25 neighbour table operations.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>