um: init cpu_tasks[] earlier

In the Linux kernel, the following vulnerability has been resolved:

um: init cpu_tasks[] earlier

This is currently done in uml_finishsetup(), but e.g. with KCOV enabled we'll crash because some init code can call into e.g. memparse(), which has coverage annotations, and then the checks in check_kcov_mode() crash because current is NULL.

Simply initialize the cpu_tasks[] array statically, which fixes the crash. For the later SMP work, it seems to have not really caused any problems yet, but initialize all of the entries anyway.