Squashfs: Rewrite and update squashfs_readahead_fragment() to not use page->index

Squashfs: Rewrite and update squashfs_readahead_fragment() to not use page->index

The previous implementation lacked error checking (e.g. the bytes
returned by squashfs_fill_page() is not checked), and the use of
page->index could not be removed without substantially rewriting
the routine to use the page actor abstraction used elsewhere.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Link: https://lore.kernel.org/r/20240818235847.170468-5-phillip@squashfs.org.uk
Signed-off-by: Christian Brauner <brauner@kernel.org>