[DECNET]: Handle a failure in neigh_parms_alloc (take 2)

While enhancing the neighbour code to handle multiple network
namespaces I noticed that decnet is assuming neigh_parms_alloc
will allways succeed, which is clearly wrong.  So handle the
failure.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Steven Whitehouse <steve@chygwyn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>