[NETFILTER_IPV4]: De-linearization of IP Connection Tracking.

[NETFILTER_IPV4]: De-linearization of IP Connection Tracking.

This converts connection tracking and all the connection tracking
modules to handle non-linear skbs.  Enough interfaces have been
broken in the process that old helpers won't compile.

Interfaces which used to take a "void *data, int len" or
"struct iphdr *iph, int len" now take the skb itself (and an offset to
the data in the case of the first interface), which is not
linearized in any way (although Alexey says after ip_rcv the IP header
is always linear, so IPv4 netfilter hooks can always assume a linear
IP hdr).

Helpers which examine data (amanda, FTP, IRC) now copy it into a buffer
and examine that.