sys/fs/tmpfs/vnd_test: make md(4) allocation dynamic

The previous logic was flawed in the sense that it assumed that /dev/md3
was always available. This was a caveat I noted in r306038, that I hadn't
gotten around to solving before now.

Cache the device for the mountpoint after executing mdmfs, then use the
cached value in basic_cleanup(..) when unmounting/disconnecting the md(4)
device.

Apply sed expressions to use reuse logic in the NetBSD code that could
also be applied to FreeBSD, just with different tools.

Differential Revision: D10766
MFC after: 1 week
Reviewed by: bdrewery
Sponsored by: Dell EMC Isilon