net/vmxnet3: Refine l2 header validation

Validation of l2 header length assumed minimal packet size as
eth_header + 2 * vlan_header regardless of the actual protocol.

This caused crash for valid non-IP packets shorter than 22 bytes, as
'tx_pkt->packet_type' hasn't been assigned for such packets, and
'vmxnet3_on_tx_done_update_stats()' expects it to be properly set.

Refine header length validation in 'vmxnet_tx_pkt_parse_headers'.
Check its return value during packet processing flow.

As a side effect, in case IPv4 and IPv6 header validation failure,
corrupt packets will be dropped.

Signed-off-by: Dana Rubin <dana.rubin@ravellosystems.com>
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
