aoe: reserve enough headroom on skbs

[ Upstream commit 91c5746425aed8f7188a351f1224a26aa232e4b3 ]

Some network drivers use a non default hard_header_len

Transmitted skb should take into account dev->hard_header_len, or risk
crashes or expensive reallocations.

In the case of aoe, lets reserve MAX_HEADER bytes.

David reported a crash in defxx driver, solved by this patch.

Reported-by: David Oostdyk <daveo@ll.mit.edu>
Tested-by: David Oostdyk <daveo@ll.mit.edu>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ed Cashin <ecashin@coraid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>