Add a request structure and make the implementation use it.

[brooks]

Summary

This allows compatibility translation to take place on the stack (md_ioctl is too big) and is more suitable as a public interface within the kernel than the kern_ioctl interface.

Except for the initialization of the md_req from the md_ioctl
(including detection of kernel md_file pointers) and the updating
of the md_ioctl prior to return, this is a mechanical replacment
of md_ioctl and mdio with md_req and mdr.

Test Plan

Passes mdconfig tests.

[cem]

sys/dev/md/md.c

1841

As long as this is a totally new ABI, this limit could be raised. If we care.

1890–1891

Will this be removed when kernel users are converted away from the ioctl API?

[brooks]

sys/dev/md/md.c

1841

I'll make the request flexible.

1890–1891

That's the plan.

[markj]

sys/dev/md/md.c

1919

Zap this stray semicolon while you're here?

[brooks]

• Whitespace fix.
• Remove unused md_base.
• Allow md_pad's length to vary.
• Change md_pad to md_units in md_req.

[brooks]

sys/dev/md/md.c

1919

@markj: I seem to have lost a race with your review and my update and I can't figure out how go get phabricator to show me where the comment should go and the current location doesn't make sense.

[brooks]

• Remove an extra semicolon.