Move 32-bit compat for md(4) ioctls into the md code.

This is more correct in that ioctl commands have no meaning until they
hit the handler associated with the file descriptor.

Add support for MDIOCRESIZE_32 which was missed when it was added.

Reviewed by: cem, kib, markj (various versions)
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14714