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 <sfr@canb.auug.org.au>
Signed-off-by: Winston Wen <wentao@uniontech.com>
Reviewed-by: Paulo Alcantara <pc@manguebit.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>