Fix up reads.

Buffer up directory read info so we can feed it to the
kernel one kdirent at a time. There is some performance
improvement to be had but this all by itself should remove the
'missed entries' problem and it also gets rid of the ghetto hack.

You should run the latest version of github.com/rminnich/go9p
to get the advantages

Signed-off-by: Ronald G. Minnich <rminnich@google.com>