KVM: x86/mmu: Allocate pml4_root for guest PAE mode shadowing

Intel inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I8UBIZ


CVE: NA

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Add the missing allocation for pml4_root. Current mmu_alloc_special_roots()
is broken, when
- L0 host is an AMD CPU, running in 64-bit mode &&
- L1 VMM is running in 32-bit/PAE mode &&
- L1 VMM is trying to create L2 guest using NPT.

The error is caused by a mistake when trying to solve the conflict, to
backport the upstreaming commit 748e52b9 ("KVM: x86/mmu: Allocate
pae_root and lm_root pages in dedicated helper"). Fix it by adding the
missing allocation of pml4_root.

Note: this error shall NOT affect any Intel platform, because an Intel
host does not need any special root:
- when EPT is disabled/un-supported, 64-bit host will use PAE shadow page
  table for 32-bit guest;
- when EPT is enabled, its level is either 4-level or 5-level, regardless
  of the guest/host paging mode, no need to allocate any special root
  either.

Fixes: cd414417 ("KVM: x86/mmu: Allocate pae_root and lm_root pages in dedicated helper")
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>