Unlock was missing on error path.

Fixes: 95f38411 ("netns: use a spin_lock to protect nsid management")
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>