Squashfs: Update squashfs_readahead() to not use page->index

Squashfs: Update squashfs_readahead() to not use page->index

This commit removes references to page->index in the pages returned
from __readahead_batch(), and instead uses the 'start' variable.

This does reveal a bug in the previous code in that 'start' was
not updated every time around the loop.  This is fixed in this
commit.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Link: https://lore.kernel.org/r/20240818235847.170468-3-phillip@squashfs.org.uk
Signed-off-by: Christian Brauner <brauner@kernel.org>