cgroup_freezer: prepare update_if_frozen() for locking change

Locking will change such that migration can happen while
freezer_read/write() is in progress.  This means that
update_if_frozen() can no longer assume that all tasks in the cgroup
coform to the current freezer state - newly migrated tasks which
haven't finished freezer_attach() yet might be in any state.

This patch updates update_if_frozen() such that it no longer verifies
task states against freezer state.  It now simply decides whether
FREEZING stage is complete.

This removal of verification makes it meaningless to call from
freezer_change_state().  Drop it and move the fast exit test from
freezer_read() - the only left caller - to update_if_frozen().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Li Zefan <lizefan@huawei.com>