btrfs: scrub: use btrfs_path::reada for extent tree readahead

For scrub, we trigger two readaheads for two trees, extent tree to get
where to scrub, and csum tree to get the data checksum.

For csum tree we already trigger readahead in
btrfs_lookup_csums_range(), by setting path->reada.
But for extent tree we don't have any path based readahead.

Add the readahead for extent tree as well, so we can later remove the
btrfs_reada_add() based readahead.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>