cache_inode_readdir: add an option to continue through the directory if a lookup fails

This completes the previous commit (allow readdir not to reach eod) so that:
 - either the option is set and readdir fails early, ganesha asks client to restart
 - the option isn't set, readdir keeps going and eod isn't reached

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
cache_inode_readdir: discriminate failure causes in populate callback & fix log levels

The only two reasons to bail early are out of memory, or trying to add dirents
to an entry that's not a directory (so a cache inode incoherence problem)

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