[1/2] kern: cpuset: allow cpuset_create() to take an allocated *setp

[kevans]

Summary

Currently, it must always allocate a new set to be used for passing to _cpuset_create, but it doesn't have to. This is purely kern_cpuset.c internal and it's sparsely used, so just change it to use *setp if it's not-NULL and modify the two consumers to pass in the address of a NULL cpuset.

This paves the way for consumers that want the unr allocation without the possibility of sleeping as long as they've done their due diligence to ensure that the mask will properly apply atop the supplied parent (i.e. avoiding the free_unr() in the last failure path).

[markj]

This seems ok to me. You might also consider renaming _cpuset_create() to cpuset_init() since the latter seems to be a better name.