KVM: arm64: Check for PTE validity when checking for executable/cacheable

KVM: arm64: Check for PTE validity when checking for executable/cacheable

Don't just assume that the PTE is valid when checking whether it
describes an executable or cacheable mapping.

This makes sure that we don't issue CMOs for invalid mappings.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240423150538.2103045-8-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>