cifs: use cifs_dirent to replace cifs_get_name_from_search_buf

This allows us to parse the on the wire structures only once in
cifs_filldir.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>