net: fix get_net_ns_by_fd for !CONFIG_NET_NS

After merging the final tree, today's linux-next build (powerpc
ppc44x_defconfig) failed like this:

net/built-in.o: In function `get_net_ns_by_fd':
(.text+0x11976): undefined reference to `netns_operations'
net/built-in.o: In function `get_net_ns_by_fd':
(.text+0x1197a): undefined reference to `netns_operations'

netns_operations is only available if CONFIG_NET_NS is set ...

Caused by commit f0630529 ("net: Allow setting the network namespace
by fd").

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>