KVM: TDX: Fold tdx_mem_page_record_premap_cnt() into its sole caller

KVM: TDX: Fold tdx_mem_page_record_premap_cnt() into its sole caller

Fold tdx_mem_page_record_premap_cnt() into tdx_sept_set_private_spte() as
providing a one-off helper for effectively three lines of code is at best a
wash, and splitting the code makes the comment for smp_rmb()  _extremely_
confusing as the comment talks about reading kvm->arch.pre_fault_allowed
before kvm_tdx->state, but the immediately visible code does the exact
opposite.

Opportunistically rewrite the comments to more explicitly explain who is
checking what, as well as _why_ the ordering matters.

No functional change intended.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@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/20251030200951.3402865-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>