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>

security/smack/smack_lsm.c

@@ -917,7 +917,7 @@ static int smack_bprm_set_creds(struct linux_binprm *bprm)
 	struct superblock_smack *sbsp;
 	int rc;
 
-	if (bprm->cred_prepared)
+	if (bprm->called_set_creds)
 		return 0;
 
 	isp = inode->i_security;