[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/nfsd/vfs.c b/fs/nfsd/vfs.c
index 84ce601..f939ba9 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1912,6 +1912,7 @@
 };
 
 struct readdir_data {
+	struct dir_context ctx;
 	char		*dirent;
 	size_t		used;
 	int		full;
@@ -1949,6 +1950,7 @@
 	int size;
 	loff_t offset;
 
+	buf.ctx.actor = nfsd_buffered_filldir;
 	buf.dirent = (void *)__get_free_page(GFP_KERNEL);
 	if (!buf.dirent)
 		return nfserrno(-ENOMEM);
@@ -1963,7 +1965,7 @@
 		buf.used = 0;
 		buf.full = 0;
 
-		host_err = vfs_readdir(file, nfsd_buffered_filldir, &buf);
+		host_err = iterate_dir(file, &buf.ctx);
 		if (buf.full)
 			host_err = 0;