block:sed-opal: SED Opal keystore

Add read and write functions that allow SED Opal keys to stored
in a permanent keystore.
Signed-off-by: Greg Joyce <gjoyce@linux.vnet.ibm.com>
Reviewed-by: Jonathan Derrick <jonathan.derrick@linux.dev>
Link: https://lore.kernel.org/r/20231004201957.1451669-2-gjoyce@linux.vnet.ibm.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/sed-opal-key.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * SED key operations.
+ *
+ * Copyright (C) 2023 IBM Corporation
+ *
+ * These are the accessor functions (read/write) for SED Opal
+ * keys. Specific keystores can provide overrides.
+ *
+ */
+
+#include <linux/kernel.h>
+
+#ifdef CONFIG_PSERIES_PLPKS_SED
+int sed_read_key(char *keyname, char *key, u_int *keylen);
+int sed_write_key(char *keyname, char *key, u_int keylen);
+#else
+static inline
+int sed_read_key(char *keyname, char *key, u_int *keylen) {
+	return -EOPNOTSUPP;
+}
+static inline
+int sed_write_key(char *keyname, char *key, u_int keylen) {
+	return -EOPNOTSUPP;
+}
+#endif