KVM: arm64: Restrict the scope of the 'writable' attribute

KVM: arm64: Restrict the scope of the 'writable' attribute

The 'writable' field is ambiguous, and indicates multiple things:

- whether the underlying memslot is writable

- whether we are resolving the fault with writable attributes

Add a new field to kvm_s2_fault_vma_info (map_writable) to indicate
the former condition, and have local writable variables to track
the latter.

Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>