KVM: x86/mmu: Allocate the lm_root before allocating PAE roots

mainline inclusion
from mainline-v5.13-rc1
commit ba0a194f
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ba0a194ffbfb4168a277fb2116e8362013e2078f



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Allocate lm_root before the PAE roots so that the PAE roots aren't
leaked if the memory allocation for the lm_root happens to fail.

Note, KVM can still leak PAE roots if mmu_check_root() fails on a guest's
PDPTR, or if mmu_alloc_root() fails due to MMU pages not being available.
Those issues will be fixed in future commits.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210305011101.3597423-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

conflict:
       arch/x86/kvm/mmu/mmu.c

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>