pk: Always set local cache header length to 4096 bytes
pk: Always set local cache header length to 4096 bytes

- No longer require the local cache header length to be a multiple of the
system page size, since we no longer need to work around loop driver
misbehavior

- Create new local cache files with a 4096-byte header to optimize for
disks with 4 KB sectors

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr