Avoid calling the underlying ->readdir() again when we reached the end
already; keep going round the loop only if we stopped due to our own
buffer being full.

[AV: tidy the things up a bit, while we are there]
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>