KVM: arm64: Extract stage-2 permission logic in user_mem_abort()

KVM: arm64: Extract stage-2 permission logic in user_mem_abort()

Extract the logic that computes the stage-2 protections and checks for
various permission faults (e.g., execution faults on non-cacheable
memory) into a new helper function, kvm_s2_fault_compute_prot(). This
helper also handles injecting atomic/exclusive faults back into the
guest when necessary.

This refactoring step separates the permission computation from the
mapping logic, making the main fault handler flow clearer.

Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>