net: reset mac header in dev_start_xmit()

On 64 bit arches :

There is a off-by-one error in qdisc_pkt_len_init() because
mac_header is not set in xmit path.

skb_mac_header() returns an out of bound value that was
harmless because hdr_len is an 'unsigned int'

On 32bit arches, the error is abysmal.

This patch is also a prereq for "macvlan: add multicast filter"

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ben Greear <greearb@candelatech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>