flat: fix uninitialized ptr with shared libs

The new credentials code broke load_flat_shared_library() as it now uses
an uninitialized cred pointer.

Reported-by: Bernd Schmidt <[e-mail]>
Tested-by: Bernd Schmidt <[e-mail]>
Cc: Mike Frysinger <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/binfmt_flat.c

···
 	if (IS_ERR(bprm.file))
 		return res;
 
+	bprm.cred = prepare_exec_creds();
+	res = -ENOMEM;
+	if (!bprm.cred)
+		goto out;
+
 	res = prepare_binprm(&bprm);
 
 	if (res <= (unsigned long)-4096)
 		res = load_flat_file(&bprm, libs, id, NULL);
-	if (bprm.file) {
-		allow_write_access(bprm.file);
-		fput(bprm.file);
-		bprm.file = NULL;
-	}
+
+	abort_creds(bprm.cred);
+
+out:
+	allow_write_access(bprm.file);
+	fput(bprm.file);
+
 	return(res);
 }