KVM: x86/mmu: Zap invalidated TDP MMU roots at 4KiB granularity

Zap invalidated TDP MMU roots at maximum granularity, i.e. with more
frequent conditional resched checkpoints, in order to avoid running for an
extended duration (milliseconds, or worse) without honoring a reschedule
request.  And for kernels running with full or real-time preempt models,
zapping at 4KiB granularity also provides significantly reduced latency
for other tasks that are contending for mmu_lock (which isn't necessarily
an overall win for KVM, but KVM should do its best to honor the kernel's
preemption model).

To keep KVM's assertion that zapping at 1GiB granularity is functionally
ok, which is the main reason 1GiB was selected in the past, skip straight
to zapping at 1GiB if KVM is configured to prove the MMU.  Zapping roots
is far more common than a vCPU replacing a 1GiB page table with a hugepage,
e.g. generally happens multiple times during boot, and so keeping the test
coverage provided by root zaps is desirable, just not for production.

Cc: David Matlack <dmatlack@google.com>
Cc: Pattara Teerapong <pteerapong@google.com>
Link: https://lore.kernel.org/r/20240111020048.844847-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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