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.comSigned-off-by: Marc Zyngier <maz@kernel.org>