packet: validate variable length ll headers

Replace link layer header validation check ll_header_truncate with
more generic dev_validate_header.

Validation based on hard_header_len incorrectly drops valid packets
in variable length protocols, such as AX25. dev_validate_header
calls header_ops.validate for such protocols to ensure correctness
below hard_header_len.

See also http://comments.gmane.org/gmane.linux.network/401064

Fixes 9c707762

 ("packet: make packet_snd fail on len smaller than l2 header")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>