KVM: TDX: Drop PROVE_MMU=y sanity check on to-be-populated mappings

Drop TDX's sanity check that a mirror EPT mapping isn't zapped between
creating said mapping and doing TDH.MEM.PAGE.ADD, as the check is
simultaneously superfluous and incomplete.  Per commit 2608f1057601
("KVM: x86/tdp_mmu: Add a helper function to walk down the TDP MMU"), the
justification for introducing kvm_tdp_mmu_gpa_is_mapped() was to check
that the target gfn was pre-populated, with a link that points to this
snippet:

 : > One small question:
 : >
 : > What if the memory region passed to KVM_TDX_INIT_MEM_REGION hasn't been pre-
 : > populated?  If we want to make KVM_TDX_INIT_MEM_REGION work with these regions,
 : > then we still need to do the real map.  Or we can make KVM_TDX_INIT_MEM_REGION
 : > return error when it finds the region hasn't been pre-populated?
 :
 : Return an error.  I don't love the idea of bleeding so many TDX details into
 : userspace, but I'm pretty sure that ship sailed a long, long time ago.

But that justification makes little sense for the final code, as the check
on nr_premapped after TDH.MEM.PAGE.ADD will detect and return an error if
KVM attempted to zap a S-EPT entry (tdx_sept_zap_private_spte() will fail
on TDH.MEM.RANGE.BLOCK due lack of a valid S-EPT entry).  And as evidenced
by the "is mapped?" code being guarded with CONFIG_KVM_PROVE_MMU=y, KVM is
NOT relying on the check for general correctness.

The sanity check is also incomplete in the sense that mmu_lock is dropped
between the check and TDH.MEM.PAGE.ADD, i.e. will only detect KVM bugs that
zap SPTEs in a very specific window (note, this also applies to the check
on nr_premapped).

Removing the sanity check will allow removing kvm_tdp_mmu_gpa_is_mapped(),
which has no business being exposed to vendor code, and more importantly
will pave the way for eliminating the "pre-map" approach entirely in favor
of doing TDH.MEM.PAGE.ADD under mmu_lock.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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