netns: remove one sparse warning

net/core/net_namespace.c:227:18: warning: incorrect type in argument 1
(different address spaces)
net/core/net_namespace.c:227:18:    expected void const *<noident>
net/core/net_namespace.c:227:18:    got struct net_generic [noderef]
<asn:4>*gen

We can use rcu_access_pointer() here as read-side access to the pointer
was removed at least one grace period ago.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>