Squashfs: fix variable overflow in squashfs_readpage_block

Squashfs: fix variable overflow in squashfs_readpage_block

Syzbot reports a slab out of bounds access in squashfs_readpage_block().

This is caused by an attempt to read page index 0x2000000000.  This value
(start_index) is stored in an integer loop variable which overflows
producing a value of 0.  This causes a loop which iterates over pages
start_index -> end_index to iterate over 0 -> end_index, which ultimately
causes an out of bounds page array access.

Fix by changing variable to a loff_t, and rename to index to make it
clearer it is a page index, and not a loop count.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Phillip Lougher <[email protected]>
Reported-by: "Lai, Yi" <[email protected]>
Closes: https://lore.kernel.org/all/ZwzcnCAosIPqQ9Ie@ly-workstation/
Signed-off-by: Andrew Morton <[email protected]>