FROMLIST: 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.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20211201170411.1561936-14-qperret@google.com
Bug: 209599700
Change-Id: I717d87c9aa2d1f1b159d7dc3bca439a2869967e5