fs: btrfs: Implement btrfs_lookup_path()

This is the extent buffer based path lookup routine.

To implement this, btrfs_lookup_dir_item() is crossported from
btrfs-progs, and implements btrfs_lookup_path() from scratch.

Unlike the existing __btrfs_lookup_path(), since btrfs_read_fs_root()
will check whether a root is a orphan at read time, there is no need to
check root backref, this makes the code a little easier to read.

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