[ARP]: Remove overkill checks from neigh_param_alloc.

Valid network device is always passed into neigh_param_alloc, so
remove extra checking for dev == NULL. Additionally, cleanup bogus
netns assignment.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>