VM_MAX_KERNEL_ADDRESS is the end of the kva range not the end of

VM_MAX_KERNEL_ADDRESS is the end of the kva range not the end of
addressable virtual memory space but arm ports were using a value of
0xffffffff for this.

Instead of using a shared VM_MAX_KERNEL_ADDRESS define add md
VM_KERNEL_SPACE_SIZE defines based on the KERNEL_VM_SIZE values
from the respective machdep.c files.

djm's novena was hitting "panic: bufinit: can't reserve VM for buffers"
without a similiar change.

ok miod@