[readdir] introduce ->iterate(), ctx->pos, dir_emit()

New method - ->iterate(file, ctx).  That's the replacement for ->readdir();
it takes callback from ctx->actor, uses ctx->pos instead of file->f_pos and
calls dir_emit(ctx, ...) instead of filldir(data, ...).  It does *not*
update file->f_pos (or look at it, for that matter); iterate_dir() does the
update.

Note that dir_emit() takes the offset from ctx->pos (and eventually
filldir_t will lose that argument).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nfsd/nfs4recover.c

@@ -240,11 +240,16 @@ struct name_list {
 	struct list_head list;
 };
 
+struct nfs4_dir_ctx {
+	struct dir_context ctx;
+	struct list_head names;
+};
+
 static int
 nfsd4_build_namelist(void *arg, const char *name, int namlen,
 		loff_t offset, u64 ino, unsigned int d_type)
 {
-	struct list_head *names = arg;
+	struct nfs4_dir_ctx *ctx = arg;
 	struct name_list *entry;
 
 	if (namlen != HEXDIR_LEN - 1)
@@ -254,7 +259,7 @@ nfsd4_build_namelist(void *arg, const char *name, int namlen,
 		return -ENOMEM;
 	memcpy(entry->name, name, HEXDIR_LEN - 1);
 	entry->name[HEXDIR_LEN - 1] = '\0';
-	list_add(&entry->list, names);
+	list_add(&entry->list, &ctx->names);
 	return 0;
 }
 
@@ -263,10 +268,7 @@ nfsd4_list_rec_dir(recdir_func *f, struct nfsd_net *nn)
 {
 	const struct cred *original_cred;
 	struct dentry *dir = nn->rec_file->f_path.dentry;
-	struct {
-		struct dir_context ctx;
-		struct list_head names;
-	} ctx;
+	struct nfs4_dir_ctx ctx;
 	int status;
 
 	status = nfs4_save_creds(&original_cred);