btrfs: fix buffer index in wait_eb_writebacks()

The commit f2cb97ee ("btrfs: index buffer_tree using node size")
changed the index of buffer_tree from "start >> sectorsize_bits" to "start
>> nodesize_bits". However, the change is not applied for
wait_eb_writebacks() and caused IO failures by writing in a full zone. Use
the index properly.

Fixes: f2cb97ee ("btrfs: index buffer_tree using node size")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>