ax25: Stop calling/abusing dev_rebuild_header

- Rename ax25_rebuild_header to ax25_neigh_xmit and call it from
  ax25_neigh_output directly.  The rename is to make it clear
  that this is not a rebuild_header operation.

- Remove ax25_rebuild_header from ax25_header_ops.

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>