The initrd_addr_max field represents the highest byte address that may
be used to hold initrd images, and is therefore almost certainly not
aligned to a page boundary: a typical value might be 0x7fffffff.

Fix the address calculations to ensure that the initrd images are
always aligned to a page boundary.

Reported-by: Sitsofe Wheeler <sitsofe@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>