sundance: Fix oopses with corrupted skb_shared_info

Joel Soete reported oopses at the beginning of pppoe connections since
v2.6.35. After debugging the bug was found in sundance skb allocation
and dma mapping code, where skb_reserve() bytes aren't taken into
account. This is an old bug, only uncovered by some change in 2.6.35.

Initial debugging patch by: Eric Dumazet <eric.dumazet@gmail.com>

Reported-by: Joel Soete <soete.joel@scarlet.be>
Tested-by: Joel Soete <soete.joel@scarlet.be>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>