callout(9): Allow spin locks use with callout_init_mtx()

[mav]

Summary

Implement lock_spin()/unlock_spin() lock class methods, moving the assertion to _sleep() instead. Change assertions in callout(9) to allow spin locks for both regular and C_DIRECT_EXEC cases. In case of C_DIRECT_EXEC spin locks are the only locks allowed.

As the first use case allow taskqueue_enqueue_timeout() use on fast task queues. It actually becomes more efficient due to avoided extra context switches in callout(9) thanks to C_DIRECT_EXEC.

Test Plan

Simple test using taskqueue_enqueue_timeout() on fast task queue, including recursively, successfully passes on debug kernel.

[• hselasky]

I like this idea and wouldn't object to it.

[mav]

Minor tuning to unlock_spin().