FROMLIST: KVM: arm64: Implement __pkvm_host_share_hyp() using do_share()

__pkvm_host_share_hyp() shares memory between the host and the
hypervisor so implement it as an invocation of the new do_share()
mechanism.

Note that double-sharing is no longer permitted (as this allows us to
reduce the number of page-table walks significantly), but is thankfully
no longer relied upon by the host.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20211201170411.1561936-13-qperret@google.com
Bug: 209599700
Change-Id: I8d44fc9ca79ac7ea5f8ca289b3cca08a4879b3cd