ax25: Refactor to use private neighbour operations.

AX25 already has it's own private arp cache operations to isolate
it's abuse of dev_rebuild_header to transmit packets.  Add a function
ax25_neigh_construct that will allow all of the ax25 devices to
force using these operations, so that the generic arp code does
not need to.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-hams@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>