debug: Fix page allocator index calculation

[bubblepipe]

The original code used usize::MAX << arch::VIRT_ADDR_BITS to calculate the base of the kernel address space. This works by coincidence for RISC-V but fails for x86_64:

For RISC-V (38-bit virtual addresses):

riscv64::KERNEL_ASPACE_BASE = 0xffffffc000000000
riscv64::VIRT_ADDR_BITS = 38
usize::MAX << riscv64::VIRT_ADDR_BITS = 0xffffffc000000000 // same as KERNEL_ASPACE_BASE

For x86_64 (48-bit virtual addresses):

x86_64::KERNEL_ASPACE_BASE = 0xffffffc000000000
x86_64::VIRT_ADDR_BITS = 48
usize::MAX << x86_64::VIRT_ADDR_BITS = 0xffff000000000000 // different with KERNEL_ASPACE_BASE

[JonasKruckenberg]

Ahh hehe, this one is on me. You must set VIRT_ADDR_BITS to one less than the "address bitness" for the math to work out. Maybe VIRT_ADDR_BITS is a bad name, but it essentially means "the highest usable bit of an address". So for RISCV Sv39 which is a 39 bit addressing system we set VIRT_ADDR_BITS = 38 and for standard x86 you should set VIRT_ADDR_BITS = 47.