firewire: net: Fix handling of fragmented multicast/broadcast packets.

This patch fixes both the transmit and receive portion of sending
fragmented mutlicast and broadcast packets.

The transmit section was broken because the offset for INTFRAG and
LASTFRAG packets were just miscalculated by IEEE1394_GASP_HDR_SIZE (which
was reserved with skb_push() in fwnet_send_packet).

The receive section was broken because in fwnet_incoming_packet is a call
to fwnet_peer_find_by_node_id(). Called with generation == -1 it will
not find a peer and the partial datagrams are associated to a peer.

[Stefan R:  The fix to use context->card->generation is not perfect.
It relies on the IR tasklet which processes packets from the prior bus
generation to run before the self-ID-complete worklet which sets the
current card generation.  Alas, there is no simple way of a race-free
implementation.  Let's do it this way for now.]

Signed-off-by: Stephan Gatzka <stephan.gatzka@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>