kern: cpuset: rename _cpuset_create() to cpuset_init()

cpuset_init() is better descriptor for what the function actually does. The
name was previously taken by a sysinit that setup cpuset_zero's mask
from all_cpus, it was removed in r331698 before stable/12 branched.

A comment referencing the removed sysinit has now also been removed, since
the setup previously done was moved into cpuset_thread0().

Suggested by: markj
MFC after: 1 week