KVM: x86/mmu: use tdp_mmu_free_sp to free roots

mainline inclusion
from mainline-v5.13-rc1
commit 4bba36d7
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=4bba36d72bf95038727d091a64dcb524dccc5da1



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Minor cleanup to deduplicate the code used to free a struct kvm_mmu_page
in the TDP MMU.

No functional change intended.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210401233736.638171-4-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>