cache_inode_readdir: handle lookup failures on readdir callback

Add an option (retry_readdir) to either:
 - Keep going if a lookup fails, and return what we found (default)
 - Ask the client to retry (JUKEBOX (nfs3), DELAY (nfs4), EAGAIN (9P)) until
we get a clean result.

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>