KVM: x86/mmu: Recover TDP MMU huge page mappings in-place instead of zapping

Recover TDP MMU huge page mappings in-place instead of zapping them when
dirty logging is disabled, and rename functions that recover huge page
mappings when dirty logging is disabled to move away from the "zap
collapsible spte" terminology.

Before KVM flushes TLBs, guest accesses may be translated through either
the (stale) small SPTE or the (new) huge SPTE. This is already possible
when KVM is doing eager page splitting (where TLB flushes are also
batched), and when vCPUs are faulting in huge mappings (where TLBs are
flushed after the new huge SPTE is installed).

Recovering huge pages reduces the number of page faults when dirty
logging is disabled:

 $ perf stat -e kvm:kvm_page_fault -- ./dirty_log_perf_test -s anonymous_hugetlb_2mb -v 64 -e -b 4g

 Before: 393,599      kvm:kvm_page_fault
 After:  262,575      kvm:kvm_page_fault

vCPU throughput and the latency of disabling dirty-logging are about
equal compared to zapping, but avoiding faults can be beneficial to
remove vCPU jitter in extreme scenarios.

Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20240823235648.3236880-5-dmatlack@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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