[NETNS]: Compilation fix for include/linux/netdevice.h.

Commit commit c346dca1
([NET] NETNS: Omit net_device->nd_net without CONFIG_NET_NS)
breaks compilation with CONFIG_NET_NS set.

Fix the typo.

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