KVM: arm64: Tolerate re-creating hyp mappings to set software bits

The current hypervisor stage-1 mapping code doesn't allow changing an
existing valid mapping. Relax this condition by allowing changes that
only target software bits, as that will soon be needed to annotate shared
pages.

Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210809152448.1810400-10-qperret@google.com