FSAL_LUSTRE: getattr: do not rely on open file remaining open without locks; use lstat anyway

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
cache_inode_readdir: allow readdir not to reach eod:

Gracefully fail if the readdir fails for some reason (external modifications),
asking the client to retry (JUKEBOX (nfs3), DELAY (nfs4), EAGAIN (9P))

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