[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/ecryptfs/file.c b/fs/ecryptfs/file.c
index a7abbea..041379a 100644
--- a/fs/ecryptfs/file.c
+++ b/fs/ecryptfs/file.c
@@ -68,6 +68,7 @@
 }
 
 struct ecryptfs_getdents_callback {
+	struct dir_context ctx;
 	void *dirent;
 	struct dentry *dentry;
 	filldir_t filldir;
@@ -126,7 +127,8 @@
 	buf.filldir = filldir;
 	buf.filldir_called = 0;
 	buf.entries_written = 0;
-	rc = vfs_readdir(lower_file, ecryptfs_filldir, (void *)&buf);
+	buf.ctx.actor = ecryptfs_filldir;
+	rc = iterate_dir(lower_file, &buf.ctx);
 	file->f_pos = lower_file->f_pos;
 	if (rc < 0)
 		goto out;