[NETNS]: Fix bad macro definition.

The macro definition is bad. When calling next_net_device with
parameter name "dev", the resulting code is:
	  struct net_device *dev = dev and that leads to an unexpected
behavior. Especially when llc_core is compiled in, the kernel panics
at boot time.
The patchset change macro definition with static inline functions as
they were defined before.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>