af_unix: netns: fix problem of return value

fix problem of return value

net/unix/af_unix.c: unix_net_init()
when error appears, it should return 'error', not always return 0.

Signed-off-by: Jianjun Kong <jianjun@zeuux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>