extensions/libxt_bpf.man: clarify BPF code generation with tcpdump

extensions/libxt_bpf.man: clarify BPF code generation with tcpdump

The xt_bpf module applies BPF bytecode to the packet. Depending on
where the module is invoked, the kernel may pass a packet with or
without link layer header. Iptables has no such header.

A common `tcpdump -ddd <string>` compilation command may revert to
a physical device that generates code for packets starting from the
mac layer up (e.g., E10MB data link type: Ethernet).

Clarify in the man page that when using this tool for code generation,
a suitable target device must be chosen.

Netfilter Bugzilla Bug #1048

Reported-by: Lorenzo Pistone <blaffablaffa@gmail.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>