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: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>