Currently, when a nested MMU is repurposed for some other MMU context,
KVM unmaps everything during vcpu_load() while holding the MMU lock for
write. This is quite a performance bottleneck for large nested VMs, as
all vCPU scheduling will spin until the unmap completes.

Start punting the MMU cleanup to a vCPU request, where it is then
possible to periodically release the MMU lock and CPU in the presence of
contention.

Ensure that no vCPU winds up using a stale MMU by tracking the pending
unmap on the S2 MMU itself and requesting an unmap on every vCPU that
finds it.
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20241007233028.2236133-4-oliver.upton@linux.devSigned-off-by: Marc Zyngier <maz@kernel.org>