FROMGIT: KVM: arm64: Implement do_unshare() helper for unsharing memory

Tearing down a previously shared memory region results in the borrower
losing access to the underlying pages and returning them to the "owned"
state in the owner.

Implement a do_unshare() helper, along the same lines as do_share(), to
provide this functionality for the host-to-hyp case.

Reviewed-by: Andrew Walbran <qwandor@google.com>
Signed-off-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/20211215161232.1480836-13-qperret@google.com
(cherry picked from commit 376a240f
 git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git next)
Bug: 209599700
Change-Id: I07aa6194099325c955f48ab608c4b601664419e5