KVM: x86/mmu: Capture 'mmu' in a local variable when allocating roots

mainline inclusion
from mainline-v5.13-rc1
commit b37233c9
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=b37233c911cbecd22a8a2a80137efe706c727d76



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Grab 'mmu' and do s/vcpu->arch.mmu/mmu to shorten line lengths and yield
smaller diffs when moving code around in future cleanup without forcing
the new code to use the same ugly pattern.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210305011101.3597423-4-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>