FROMGIT: KVM: arm64: pkvm: Refcount the pages shared with EL2

In order to simplify the page tracking infrastructure at EL2 in nVHE
protected mode, move the responsibility of refcounting pages that are
shared multiple times on the host. In order to do so, let's create a
red-black tree tracking all the PFNs that have been shared, along with
a refcount.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211215161232.1480836-8-qperret@google.com
(cherry picked from commit a83e2191
 git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git next)
Bug: 209599700
Change-Id: I58f39d139c84acb545676c99144547cd080cdda3