fs: btrfs: Use btrfs_iter_dir() to replace btrfs_readdir()

Use extent buffer based infrastructure to re-implement btrfs_readdir().

Along this rework, some small corner cases fixed:
- Subvolume tree mtime
  Mtime of a subvolume tree is recorded in its root item, since there is
  no INODE_ITEM for it.
  This needs extra search from tree root.

- Output the unknown type
  If the DIR_ITEM is corrupted, at least don't try to access the memory
  out of boundary.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>