Add open_dir_by_path_walk for safe implementation of lookup_path

FSAL lookup_path implementations (which are called to get the root
handle of an export at init time AND for v3 mounts of a sub-directory of
the export) are vulnerable to two attacks:

- First, if a symlink is in the path other than as the last element, the
old code does not reject the lookup.

- Second, and worse, the .. entry is not looked for...

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
TEMP

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>