KVM: arm64: Make table->block changes parallel-aware

stage2_map_walker_try_leaf() and friends now handle stage-2 PTEs
generically, and perform the correct flush when a table PTE is removed.
Additionally, they've been made parallel-aware, using an atomic break
to take ownership of the PTE.

Stop clearing the PTE in the pre-order callback and instead let
stage2_map_walker_try_leaf() deal with it.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221107220006.1895572-1-oliver.upton@linux.dev