Move common fields for queue management to struct nf_info and rename it
to struct nf_queue_entry. The avoids one allocation/free per packet and
simplifies the code a bit.

Alternatively we could add some private room at the tail, but since
all current users use identical structs this seems easier.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>