fs: always use container's actual size not requested size

The requested size may not match the final size of the container,
so consistently use the container's size after prepare(), not the
requested size.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/fs.cc

@@ -409,7 +409,7 @@ namespace crucible {
 		btrfs_ioctl_ino_path_args *p = static_cast<btrfs_ioctl_ino_path_args *>(this);
 		BtrfsDataContainer container(m_container_size);
 		fspath = reinterpret_cast<uint64_t>(container.prepare(m_container_size));
-		size = m_container_size;
+		size = container.get_size();
 
 		m_paths.clear();