btrfs: handle errors from btrfs_dec_ref() properly

commit 5eb178f3 upstream.

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>
Signed-off-by: Jianqi Ren <jianqi.ren.cn@windriver.com>
Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>