cgroup_freezer: simplify propagation of CGROUP_FROZEN clearing in freezer_attach()

If one or more tasks get moved into a frozen css, the frozen state is
cleared up from the destination css so that it can be reasserted once
the migrated tasks are frozen.  freezer_attach() implements this in
two separate steps - clearing CGROUP_FROZEN on the target css while
processing each task and propagating the clearing upwards after the
task loop is done if necessary.

This patch merges the two steps.  Propagation now takes place inside
the task loop.  This simplifies the code and prepares it for the fix
of multi-destination migration.

Signed-off-by: Tejun Heo <tj@kernel.org>