fs/nls: make load_nls() take a const parameter

load_nls() take a char * parameter, use it to find nls module in list or
construct the module name to load it.

This change make load_nls() take a const parameter, so we don't need do
some cast like this:

ses->local_nls = load_nls((char *)ctx->local_nls->charset);

Suggested-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Winston Wen <[e-mail]>
Reviewed-by: Paulo Alcantara <[e-mail]>
Reviewed-by: Christian Brauner <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/nls/nls_base.c

···
 	return -EINVAL;
 }
 
-static struct nls_table *find_nls(char *charset)
+static struct nls_table *find_nls(const char *charset)
 {
 	struct nls_table *nls;
 	spin_lock(&nls_lock);
···
 	return nls;
 }
 
-struct nls_table *load_nls(char *charset)
+struct nls_table *load_nls(const char *charset)
 {
 	return try_then_request_module(find_nls(charset), "nls_%s", charset);
 }

include/linux/nls.h

···
 /* nls_base.c */
 extern int __register_nls(struct nls_table *, struct module *);
 extern int unregister_nls(struct nls_table *);
-extern struct nls_table *load_nls(char *);
+extern struct nls_table *load_nls(const char *charset);
 extern void unload_nls(struct nls_table *);
 extern struct nls_table *load_nls_default(void);
 #define register_nls(nls) __register_nls((nls), THIS_MODULE)