ARM: KVM: fix compilation after removal of user_alloc from struct kvm_memory_slot

Commit 7a905b14 (KVM: Remove user_alloc from struct kvm_memory_slot)
broke KVM/ARM by removing the user_alloc field from a public structure.

As we only used this field to alert the user that we didn't support
this operation mode, there is no harm in discarding this bit of code
without any remorse.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>