cifs: clean up id_mode_to_cifs_acl

Add a label we can goto on error, and get rid of some excess indentation.
Also move to kernel-style comments.

Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>