passthrough: reduce the memory footprint of file handle

[liubogithub]

File handles used by name_to_handle_at(2) are allocated with fixed max bytes, which results in unnecessary memory pressure.

Per the manual of open_by_handle_at(2),

"The caller can discover the required size for the file_handle structure by making a call in which handle->handle_bytes is zero; in this case, the call fails with the error EOVERFLOW and handle->handle_bytes is set to indicate the required size; the caller can then use this information to allocate a structure of the correct size (see EXAMPLES below)."

This follows the description and allocates the needed bytes.

With this,
sizeof:: drops from 136 bytes to 16 bytes.
sizeof:: drops from 328 bytes to 120 bytes.

Also there is no need to make CFileHandle public.

Signed-off-by: Liu Bo [email protected]

Test has been done with passthrough::tests::test_passthroughfs_inode_file_handles (with the following changes and tempdir is on ext4 which supports file handle feature),

@@ -1330,8 +1331,8 @@ mod tests {
         let c_entry = fs.lookup(&ctx, p_inode, &child).unwrap();

         // Following test depends on host fs, it's not reliable.
-        //let data = fs.inode_map.get(c_entry.inode).unwrap();
-        //assert_eq!(matches!(data.file_or_handle, FileOrHandle::Handle(_)), true);
+        let data = fs.inode_map.get(c_entry.inode).unwrap();
+        assert_eq!(matches!(data.file_or_handle, FileOrHandle::Handle(_)), true);

         let (_, duration) = fs.getattr(&ctx, c_entry.inode, None).unwrap();
         assert_eq!(duration, fs.cfg.attr_timeout);