KVM/TDX: Allocate PAMT memory for TD and vCPU control structures

Use control page helpers for allocating and freeing TD control structures,
such that these operations can work for Dynamic PAMT.

The TDX module tracks some state for each page of physical memory that it
might use. It calls this state the PAMT. It includes separate state for
each page size a physical page could be utilized at within the TDX module
(1GB, 2MB, 4KB). In Dynamic PAMT, only the 4KB page size state is
allocated dynamically. So the kernel must ensure PAMT backing is installed
for any 4KB page being gifted to the TDX module, and must tear down the
backing when all associated gifted pages are reclaimed.

TD scoped control pages (TDR, TDCS) and vCPU scoped control pages (TDVPR,
TDCX) are all handed to the TDX module at 4KB page size and are therefore
subject to this requirement. Replace the raw alloc_page()/__free_page()
calls for these pages with tdx_alloc/free_control_page().

Switching between special Dynamic PAMT operations or normal page
alloc/free operations is handled internally in
tdx_alloc/free_control_page(). So don't check for Dynamic PAMT around these
calls. Just call them unconditionally. Similarly, drop the NULL checks
before freeing, as tdx_free_control_page() handles NULL internally.

No functional change intended when DPAMT is not in use.

[sean: handle alloc+free+reclaim in one patch]
[rick: enhance log, reviewing, rebase, with help from AI tooling]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Vishal Annapurve <vannapurve@google.com>
Acked-by: Sohil Mehta <sohil.mehta@intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Tested-by: Hongyu Ning <hongyu.ning@linux.intel.com>
Link: https://patch.msgid.link/20260904215841.303070-7-rick.p.edgecombe@intel.com

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