btrfs: handle errors from btrfs_dec_ref() properly

In walk_up_proc() we BUG_ON(ret) from btrfs_dec_ref().  This is
incorrect, we have proper error handling here, return the error.

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