em_ipset: use dev_net() accessor

Randy found that if network namespace not enabled then
nd_net does not exist and would cause compilation failure.

This is handled correctly by using the dev_net() macro.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>