cgroup: rename freezer.c into legacy_freezer.c

Freezer.c will contain an implementation of cgroup v2 freezer,
so let's rename the v1 freezer to avoid naming conflicts.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com