whole KVA space using one locked 4MB dTLB entry per GB of physical
memory. On Cheetah-class machines only the dt16 can hold locked
entries though, which would be completely consumed for the kernel
TSB on machines with >= 16GB. Therefore limit the KVA space to use
no more than half of the lockable dTLB slots, given that we need
them also for other things.
- Add sanity checks which ensure that we don't exhaust the (lockable)
TLB slots.
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