attach: allow LSM attach without new mnt namespace

attach: allow LSM attach without new mnt namespace

Currently, the -c command (to set the selinux context) seems to be
broken because lxc-attach expects that also a new mount namespace
is specified via command line. This commit remove the check for the new
mount namespace to fix this issue. Please note that the
--elevated-privileges option is not affected by this issue.

Signed-off-by: Maximilian Blenk <Maximilian.Blenk@bmw.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>