Fixes short 9ns dirent reads

Things like 'ps' were failing.  There might have been others.  The read
method would work, but it was possible to squeeze in multiple Ms in the
buffer for a single kdirent read.  End result, we'd miss entries every
so often.  For devproc, we'd miss the second of three processes.

This didn't take as long to figure this out as when it happened to
devmnt, but still took took long.

I put in a check in devdirread which should trip if our minimum is wrong
again.