KVM: TDX: Move external page table freeing to TDX code

Move the freeing of external page tables into the reclaim operation that
lives in TDX code.

The TDP MMU supports traversing the TDP without holding locks. Page tables
need to be freed via RCU to prevent walking one that gets freed.

While none of these lockless walk operations actually happen for the mirror
page table, the TDP MMU nonetheless frees the mirror page table in the same
way, and (because it's a handy place to plug it in) the external page table
as well.

However, the external page table definitely can't be walked once the page
table pages are reclaimed from the TDX module. The TDX module releases the
page for the host VMM to use, so this RCU-time free is unnecessary for the
external page table.

So move the free_page() call to TDX code. Create an
tdp_mmu_free_unused_sp() to allow for freeing external page tables that
have never left the TDP MMU code (i.e. don't need to be freed in a special
way).

Link: https://lore.kernel.org/kvm/aYpjNrtGmogNzqwT@google.com
[Based on a diff by Sean, added log]
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260509075740.4371-1-yan.y.zhao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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