KVM: x86/mmu: Demote the WARN on yielded in xxx_cond_resched() to KVM_MMU_WARN_ON

Convert the WARN in tdp_mmu_iter_cond_resched() that the iterator hasn't
already yielded to a KVM_MMU_WARN_ON() so the code is compiled out for
production kernels (assuming production kernels disable KVM_PROVE_MMU).

Checking for a needed reschedule is a hot path, and KVM sanity checks
iter->yielded in several other less-hot paths, i.e. the odds of KVM not
flagging that something went sideways are quite low.  Furthermore, the
odds of KVM not noticing *and* the WARN detecting something worth
investigating are even lower.

Link: https://lore.kernel.org/r/20241031170633.1502783-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>