KVM: arm64: Return EAGAIN for invalid PTE in attr walker

Return EAGAIN for invalid PTEs in the attr walker, signaling to the
caller that any serialization and/or invalidation can be elided.

Link: https://lore.kernel.org/r/20221202185156.696189-4-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>