container freezer: prevent frozen tasks or cgroups from changing

Don't let frozen tasks or cgroups change.  This means frozen tasks can't
leave their current cgroup for another cgroup.  It also means that tasks
cannot be added to or removed from a cgroup in the FROZEN state.  We
enforce these rules by checking for frozen tasks and cgroups in the
can_attach() function.

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>