Revert "btrfs: switch to ->iterate_shared()"

This reverts commit 972b241.
Quoth Chris:
	didn't take the delayed inode stuff into account
	it got an rbtree of items and it pulls things out
	so in shared mode, its hugely racey
	sorry, lets revert and fix it for real inside of btrfs

Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/btrfs/inode.c

@@ -10181,7 +10181,7 @@ static const struct inode_operations btrfs_dir_ro_inode_operations = {
 static const struct file_operations btrfs_dir_file_operations = {
 	.llseek		= generic_file_llseek,
 	.read		= generic_read_dir,
-	.iterate_shared	= btrfs_real_readdir,
+	.iterate	= btrfs_real_readdir,
 	.unlocked_ioctl	= btrfs_ioctl,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	= btrfs_ioctl,