[readdir] introduce iterate_dir() and dir_context

iterate_dir(): new helper, replacing vfs_readdir().

struct dir_context: contains the readdir callback (and will get more stuff
in it), embedded into whatever data that callback wants to deal with;
eventually, we'll be passing it to ->readdir() replacement instead of
(data,filldir) pair.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 262fc99..7cb1904 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -212,6 +212,7 @@
 }
 
 struct getdents_callback {
+	struct dir_context ctx;
 	char *name;		/* name that was found. It already points to a
 				   buffer NAME_MAX+1 is size */
 	unsigned long ino;	/* the inum we are looking for */
@@ -278,10 +279,11 @@
 	buffer.ino = child->d_inode->i_ino;
 	buffer.found = 0;
 	buffer.sequence = 0;
+	buffer.ctx.actor = filldir_one;
 	while (1) {
 		int old_seq = buffer.sequence;
 
-		error = vfs_readdir(file, filldir_one, &buffer);
+		error = iterate_dir(file, &buffer.ctx);
 		if (buffer.found) {
 			error = 0;
 			break;