netfilter: possible unaligned packet header in ip_route_me_harder

This patch tries to fix the following issue in netfilter:
In ip_route_me_harder(), we invoke pskb_expand_head() that
rellocates new header with additional head room which can break
the alignment of the original packet header.

In one of my NAT test case, the NIC port for internal hosts is
configured with vlan and the port for external hosts is with
general configuration. If we ping an external "unknown" hosts from an
internal host, an icmp packet will be sent. We find that in
icmp_send()->...->ip_route_me_harder()->pskb_expand_head(), hh_len=18
and current headroom (skb_headroom(skb)) of the packet is 16. After
calling pskb_expand_head() the packet header becomes to be unaligned
and then our system (arch/tile) panics immediately.

Signed-off-by: Paul Guo <ggang@tilera.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>