Don't try to thaw processes when we haven't frozen them

The thaw_processes function now checks that the caller was
the same cpu that froze processes, by looking for PF_SUSPEND_TASK
flag. If this isn't set, it triggers a WARN_ON. To avoid this,
add a check that PF_SUSPEND_TASK is set before calling the function.

Signed-off-by: Nigel Cunningham <nigel@tuxonice.net>