KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make

The break-before-make sequence is a bit annoying as it opens a window
wherein memory is unmapped from the guest. KVM should replace the PTE
as quickly as possible and avoid unnecessary work in between.

Presently, the stage-2 map walker tears down a removed table before
installing a block mapping when coalescing a table into a block. As the
removed table is no longer visible to hardware walkers after the
DSB+TLBI, it is possible to move the remaining cleanup to happen after
installing the new PTE.

Reshuffle the stage-2 map walker to install the new block entry in
the pre-order callback. Unwire all of the teardown logic and replace
it with a call to kvm_pgtable_stage2_free_removed() after fixing
the PTE. The post-order visitor is now completely unnecessary, so drop
it. Finally, touch up the comments to better represent the now
simplified map walker.

Note that the call to tear down the unlinked stage-2 is indirected
as a subsequent change will use an RCU callback to trigger tear down.
RCU is not available to pKVM, so there is a need to use different
implementations on pKVM and non-pKVM VMs.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221107215644.1895162-8-oliver.upton@linux.dev