[NETFILTER]: nfnetlink_queue: eliminate impossible switch case

We don't need a default case in nfqnl_build_packet_message(), the
copy_mode is validated when it is set. Tell the compiler about
the possible types and remove the default case. Saves 80b of
text on x86_64.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>