PM / Freezer: Remove the "userspace only" constraint from freezer[_do_not]_count()

At present, the functions freezer_count() and freezer_do_not_count()
impose the restriction that they are effective only for userspace processes.
However, now, these functions have found more utility than originally
intended by the commit which introduced it: ba96a0c8

 (freezer:
fix vfork problem). And moreover, even the vfork issue actually does not
need the above restriction in these functions.

So, modify these functions to make them work even for kernel threads, so
that they can be used at other places in the kernel, where the userspace
restriction doesn't apply.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Tejun Heo <tj@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>