exec: Rename bprm->cred_prepared to called_set_creds

The cred_prepared bprm flag has a misleading name. It has nothing to do
with the bprm_prepare_cred hook, and actually tracks if bprm_set_creds has
been called. Rename this flag and improve its comment.

Cc: David Howells <dhowells@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: John Johansen <john.johansen@canonical.com>
Acked-by: James Morris <james.l.morris@oracle.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Acked-by: Serge Hallyn <serge@hallyn.com>

fs/binfmt_flat.c

@@ -890,7 +890,7 @@ static int load_flat_shared_library(int id, struct lib_info *libs)
 	 * as we're past the point of no return and are dealing with shared
 	 * libraries.
 	 */
-	bprm.cred_prepared = 1;
+	bprm.called_set_creds = 1;
 
 	res = prepare_binprm(&bprm);