MFC r307567 (by sbruno): Assert that we're assigning a non-null taskqueue.
ref: https://github.com/NextBSD/NextBSD/commit/535865d02c162e415d7436899cd6db5000a0cc7b

Fix cpu assignment by assuring stride is non-zero, assert that all tasks
have a valid taskqueue.
ref: https://github.com/NextBSD/NextBSD/commit/db398176234fe3ce9f8e8b671f56000f8276feba

Start cpu assignment from zero.
ref: https://github.com/NextBSD/NextBSD/commit/d99d39b6b6c5dfac1eb440c41e36ebf4c897198e