Btrfs: return free space in cow error path

Replace some BUG_ONs with proper handling and take allocated space back to
free space cache for later use.

We don't have to worry about extent maps since they'd be freed in releasepage
path.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/inode.c

@@ -920,7 +920,8 @@ static noinline int __cow_file_range(struct btrfs_trans_handle *trans,
 		}
 
 		em = alloc_extent_map();
-		BUG_ON(!em); /* -ENOMEM */
+		if (!em)
+			goto out_reserve;
 		em->start = start;
 		em->orig_start = em->start;
 		ram_size = ins.offset;
@@ -947,19 +948,22 @@ static noinline int __cow_file_range(struct btrfs_trans_handle *trans,
 			btrfs_drop_extent_cache(inode, start,
 						start + ram_size - 1, 0);
 		}
+		if (ret)
+			goto out_reserve;
 
 		cur_alloc_size = ins.offset;
 		ret = btrfs_add_ordered_extent(inode, start, ins.objectid,
 					       ram_size, cur_alloc_size, 0);
-		BUG_ON(ret); /* -ENOMEM */
+		if (ret)
+			goto out_reserve;
 
 		if (root->root_key.objectid ==
 		    BTRFS_DATA_RELOC_TREE_OBJECTID) {
 			ret = btrfs_reloc_clone_csums(inode, start,
 						      cur_alloc_size);
 			if (ret) {
 				btrfs_abort_transaction(trans, root, ret);
-				goto out_unlock;
+				goto out_reserve;
 			}
 		}
 
@@ -988,6 +992,8 @@ static noinline int __cow_file_range(struct btrfs_trans_handle *trans,
 out:
 	return ret;
 
+out_reserve:
+	btrfs_free_reserved_extent(root, ins.objectid, ins.offset);
 out_unlock:
 	extent_clear_unlock_delalloc(inode,
 		     &BTRFS_I(inode)->io_tree,