netns 1/2: don't get/put old netns on CLONE_NEWNET

copy_net_ns() doesn't copy anything, it creates fresh netns, so
get/put of old netns isn't needed.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>