cifs: set the CREATE_NOT_FILE when opening the directory in use_cached_dir()

This enforces that we can only do this for directories and not normal files
or else the server will return an error.
This means that we will have conditionally check IF the path refers
to a directory or not in all the call-sites where we are unsure.
Right now this check is for "" i.e. root.

Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>