KVM: arm64: Directly expose mapping prot and kill kvm_s2_fault

KVM: arm64: Directly expose mapping prot and kill kvm_s2_fault

The 'prot' field is the only one left in kvm_s2_fault. Expose it
directly to the functions needing it, and get rid of kvm_s2_fault.

It has served us well during this refactoring, but it is now no
longer needed.

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>