KVM: x86/mmu: Allocate pae_root and lm_root pages in dedicated helper

mainline inclusion
from mainline-v5.13-rc1
commit 748e52b9
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=748e52b9b7368017d3fccb486914804ed4577b42



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Move the on-demand allocation of the pae_root and lm_root pages, used by
nested NPT for 32-bit L1s, into a separate helper.  This will allow a
future patch to hold mmu_lock while allocating the non-special roots so
that make_mmu_pages_available() can be checked once at the start of root
allocation, and thus avoid having to deal with failure in the middle of
root allocation.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210305011101.3597423-6-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>