KVM: arm64: Commit pending PC adjustemnts before returning to userspace

KVM currently updates PC (and the corresponding exception state)
using a two phase approach: first by setting a set of flags,
then by converting these flags into a state update when the vcpu
is about to enter the guest.

However, this creates a disconnect with userspace if the vcpu thread
returns there with any exception/PC flag set. In this case, the exposed
context is wrong, as userspace doesn't have access to these flags
(they aren't architectural). It also means that these flags are
preserved across a reset, which isn't expected.

To solve this problem, force an explicit synchronisation of the
exception state on vcpu exit to userspace. As an optimisation
for nVHE systems, only perform this when there is something pending.

Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Tested-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org # 5.11