btrfs: change BUG_ON to assertion when verifying root in btrfs_alloc_reserved_file_extent()

The file extents are normally reserved in subvolume roots but could be
also in the data reloc tree. Change the BUG_ON to assertions as this
verifies the usage assumptions.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -4961,7 +4961,7 @@ int btrfs_alloc_reserved_file_extent(struct btrfs_trans_handle *trans,
 	u64 root_objectid = root->root_key.objectid;
 	u64 owning_root = root_objectid;
 
-	BUG_ON(root_objectid == BTRFS_TREE_LOG_OBJECTID);
+	ASSERT(root_objectid != BTRFS_TREE_LOG_OBJECTID);
 
 	if (btrfs_is_data_reloc_root(root) && is_fstree(root->relocation_src_root))
 		owning_root = root->relocation_src_root;