Reduce bufdaemon/bufspacedaemon shutdown time

[mav]

Summary

Before this change bufdaemon and bufspacedaemon threads used kthread_shutdown() to stop activity on system shutdown. The problem is that kthread_shutdown() has no idea about the wait channel and lock used by specific thread to wake them up reliably. As result, up to 9 threads could consume up to 9 seconds to shutdown for no good reason.

This change introduces specific shutdown functions, knowing how to properly wake up specific threads, reducing wait for those threads on shutdown/reboot from average 4 seconds to effectively zero.

Test Plan

Confirmed much faster reboots on both VM and hardware systems.

[mav]

Rename _exit() into _shutdown().

[mav]

Rename variables also.

[markj]

Nice, thanks.

sys/kern/vfs_bio.c

768

799

3411

3456

[mav]

It would be good if somebody reviewed what "syncer" does while printing zeroes for 4 seconds on shutdown/reboot. I have suspicion that it just wastes time for NFS or ZFS roots. It annoys me for years.