freezer: don't unnecessarily set PF_NOFREEZE explicitly

freezer: don't unnecessarily set PF_NOFREEZE explicitly

Some drivers set PF_NOFREEZE in their kthread functions which is
completely unnecessary and racy - some part of freezer code doesn't
consider cases where PF_NOFREEZE is set asynchronous to freezer
operations.

In general, there's no reason to allow setting PF_NOFREEZE explicitly.
Remove them and change the documentation to note that setting
PF_NOFREEZE directly isn't allowed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Gustavo F. Padovan" <padovan@profusion.mobi>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: wwang <wei_wang@realsil.com.cn>