firewire: net: fix unicast reception RCODE in failure paths

The incoming request hander fwnet_receive_packet() expects subsequent
datagram handling code to return non-zero on errors.  However, almost
none of the failure paths did so.  Fix them all.

(This error reporting is used to send and RCODE_CONFLICT_ERROR to the
sender node in such failure cases.  Two modes of failure exist:  Out of
memory, or firewire-net is unaware of any peer node to which a fragment
or an ARP packet belongs.  However, it is unclear whether a sender can
actually make use of such information.  A Linux peer apparently can't.
Maybe it should all be simplified to void functions.)

Reported-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>