neigh: use neigh_parms_net() to get struct neigh_parms->net pointer

This fixes compile error when CONFIG_NET_NS is not set.

Introduced by:
commit 1d4c8c29
    "neigh: restore old behaviour of default parms values"

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>