btrfs: return errors from unpin_extent_range()

Handle the lookup failure of the block group to unpin, this is a logic
error as the block group must exist at this point. If not, something else
must have freed it, like clean_pinned_extents() would do without locking
the unused_bg_unpin_mutex.

Push the errors to the callers, proper handling will be done in followup
patches.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -1429,7 +1429,7 @@ static bool clean_pinned_extents(struct btrfs_trans_handle *trans,
 	 * group in pinned_extents before we were able to clear the whole block
 	 * group range from pinned_extents. This means that task can lookup for
 	 * the block group after we unpinned it from pinned_extents and removed
-	 * it, leading to a BUG_ON() at unpin_extent_range().
+	 * it, leading to an error at unpin_extent_range().
 	 */
 	mutex_lock(&fs_info->unused_bg_unpin_mutex);
 	if (prev_trans) {

fs/btrfs/extent-tree.c

@@ -2777,6 +2777,7 @@ static int unpin_extent_range(struct btrfs_fs_info *fs_info,
 	u64 total_unpinned = 0;
 	u64 empty_cluster = 0;
 	bool readonly;
+	int ret = 0;
 
 	while (start <= end) {
 		readonly = false;
@@ -2786,7 +2787,11 @@ static int unpin_extent_range(struct btrfs_fs_info *fs_info,
 				btrfs_put_block_group(cache);
 			total_unpinned = 0;
 			cache = btrfs_lookup_block_group(fs_info, start);
-			BUG_ON(!cache); /* Logic error */
+			if (cache == NULL) {
+				/* Logic error, something removed the block group. */
+				ret = -EUCLEAN;
+				goto out;
+			}
 
 			cluster = fetch_cluster_info(fs_info,
 						     cache->space_info,
@@ -2855,7 +2860,8 @@ static int unpin_extent_range(struct btrfs_fs_info *fs_info,
 
 	if (cache)
 		btrfs_put_block_group(cache);
-	return 0;
+out:
+	return ret;
 }
 
 int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans)
@@ -2885,7 +2891,8 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans)
 						   end + 1 - start, NULL);
 
 		clear_extent_dirty(unpin, start, end, &cached_state);
-		unpin_extent_range(fs_info, start, end, true);
+		ret = unpin_extent_range(fs_info, start, end, true);
+		BUG_ON(ret);
 		mutex_unlock(&fs_info->unused_bg_unpin_mutex);
 		free_extent_state(cached_state);
 		cond_resched();
@@ -6167,7 +6174,11 @@ int btrfs_drop_subtree(struct btrfs_trans_handle *trans,
 int btrfs_error_unpin_extent_range(struct btrfs_fs_info *fs_info,
 				   u64 start, u64 end)
 {
-	return unpin_extent_range(fs_info, start, end, false);
+	int ret;
+
+	ret = unpin_extent_range(fs_info, start, end, false);
+	BUG_ON(ret);
+	return ret;
 }
 
 /*