CVE-2017-5985: Ensure target netns is caller-owned

Before this commit, lxc-user-nic could potentially have been tricked into
operating on a network namespace over which the caller did not hold privilege.

This commit ensures that the caller is privileged over the network namespace by
temporarily dropping privilege.

Launchpad: https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1654676
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>