btrfs: handle errors in walk_down_tree properly

We can get errors in walk_down_proc as we try and lookup extent info for
the snapshot dropping to act on.  However if we get an error we simply
return 1 which indicates we're done with walking down, which will lead
us to improperly continue with the snapshot drop with the incorrect
information.  Instead break if we get any error from walk_down_proc or
do_walk_down, and handle the case of ret == 1 by returning 0, otherwise
return the ret value that we have.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>