unicode: implement higher level API for string handling

This patch integrates the utf8n patches with some higher level API to
perform UTF-8 string comparison, normalization and casefolding
operations.  Implemented is a variation of NFD, and casefold is
performed by doing full casefold on top of NFD.  These algorithms are
based on the core implemented by Olaf Weber from SGI.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

include/linux/unicode.h

@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_UNICODE_H
+#define _LINUX_UNICODE_H
+
+#include <linux/init.h>
+#include <linux/dcache.h>
+
+struct unicode_map {
+	const char *charset;
+	int version;
+};
+
+int utf8_validate(const struct unicode_map *um, const struct qstr *str);
+
+int utf8_strncmp(const struct unicode_map *um,
+		 const struct qstr *s1, const struct qstr *s2);
+
+int utf8_strncasecmp(const struct unicode_map *um,
+		 const struct qstr *s1, const struct qstr *s2);
+
+int utf8_normalize(const struct unicode_map *um, const struct qstr *str,
+		   unsigned char *dest, size_t dlen);
+
+int utf8_casefold(const struct unicode_map *um, const struct qstr *str,
+		  unsigned char *dest, size_t dlen);
+
+struct unicode_map *utf8_load(const char *version);
+void utf8_unload(struct unicode_map *um);
+
+#endif /* _LINUX_UNICODE_H */