md(4): Direct mode

[trasz]

Summary

This makes it possible for md(4) to avoid using a separate thread
to perform IO, instead doing it all immediately within the GEOM
thread. Speeds up diskinfo -Sw about 4x for really tiny IO,
otherwise makes things slower. Not a commit candidate; stashing
here for future reference.

[darkfiberiru_gmail.com]

What is really tiny i/o? Smaller then what?

[trasz]

The 4x difference was for 512B-size writes.