btrfs: replace BUG() with error handling in __btrfs_balance()

We search with offset (u64)-1 which should never match exactly.
Previously this was handled with BUG(). Now logs an error
and return -EUCLEAN.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Adarsh Das <adarshdas950@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: Saphereye

fs/btrfs/volumes.c

@@ -4367,8 +4367,14 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
 		 * this shouldn't happen, it means the last relocate
 		 * failed
 		 */
-		if (ret == 0)
-			BUG(); /* FIXME break ? */
+		if (unlikely(ret == 0)) {
+			btrfs_err(fs_info,
+				  "unexpected exact match of CHUNK_ITEM in chunk tree, offset 0x%llx",
+				  key.offset);
+			mutex_unlock(&fs_info->reclaim_bgs_lock);
+			ret = -EUCLEAN;
+			goto error;
+		}
 
 		ret = btrfs_previous_item(chunk_root, path, 0,
 					  BTRFS_CHUNK_ITEM_KEY);