KVM: x86/mmu: Move kvm_mmu_(get|put)_root to TDP MMU

mainline inclusion
from mainline-v5.13-rc1
commit 76eb54e7
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=76eb54e7e717455b4c5b82cec5c879ea017610f5



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The TDP MMU is almost the only user of kvm_mmu_get_root and
kvm_mmu_put_root. There is only one use of put_root in mmu.c for the
legacy / shadow MMU. Open code that one use and move the get / put
functions to the TDP MMU so they can be extended in future commits.

No functional change intended.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210401233736.638171-3-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>