KVM: x86/mmu: Factor out functions to add/remove TDP MMU pages

mainline inclusion
from mainline-v5.12-rc1
commit a9442f59
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=a9442f594147f95307f691cfba0c31e25dc79b9d



----------------------------------------------------------------------

Move the work of adding and removing TDP MMU pages to/from  "secondary"
data structures to helper functions. These functions will be built on in
future commits to enable MMU operations to proceed (mostly) in parallel.

No functional change expected.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210202185734.1680553-20-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>