KVM: x86/mmu: WARN on any attempt to atomically update REMOVED SPTE

Disallow calling tdp_mmu_set_spte_atomic() with a REMOVED "old" SPTE.
This solves a conundrum introduced by commit 3255530ab191 ("KVM: x86/mmu:
Automatically update iter->old_spte if cmpxchg fails"); if the helper
doesn't update old_spte in the REMOVED case, then theoretically the
caller could get stuck in an infinite loop as it will fail indefinitely
on the REMOVED SPTE.  E.g. until recently, clear_dirty_gfn_range() didn't
check for a present SPTE and would have spun until getting rescheduled.

In practice, only the page fault path should "create" a new SPTE, all
other paths should only operate on existing, a.k.a. shadow present,
SPTEs.  Now that the page fault path pre-checks for a REMOVED SPTE in all
cases, require all other paths to indirectly pre-check by verifying the
target SPTE is a shadow-present SPTE.

Note, this does not guarantee the actual SPTE isn't REMOVED, nor is that
scenario disallowed.  The invariant is only that the caller mustn't
invoke tdp_mmu_set_spte_atomic() if the SPTE was REMOVED when last
observed by the caller.

Cc: David Matlack <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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