freezer: rename thaw_process() to __thaw_task() and simplify the implementation

freezer: rename thaw_process() to __thaw_task() and simplify the implementation

thaw_process() now has only internal users - system and cgroup
freezers.  Remove the unnecessary return value, rename, unexport and
collapse __thaw_process() into it.  This will help further updates to
the freezer code.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Paul Menage <menage@google.com>