In the help text describing user namespaces recommend use of memory
control groups.  In many cases memory control groups are the only
mechanism there is to limit how much memory a user who can create
user namespaces can use.
Acked-by: NSerge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>