sandbox/fs: Make linking of nodes in os_dirent_ls more obvious

Previously, after reading/creating the second dirent, the second entry
would be chained to the first entry and the first entry would be linked
to head. Instead, immediately link the first entry to head.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Simon Glass <sjg@chromium.org>