BACKPORT: 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
Change-Id: Ie196fbcca1e0bf46af9200752d8fdf90b97e5a8b
(cherry picked from commit 50943f3e136adfc421f9768d6ae09ba7b83aaefd)
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>