Let's introduce an explicit check if skeys have already been enabled
for the vcpu, so we don't have to check the mm context if we don't have
the storage key facility.

This lets us check for enablement without having to take the mm
semaphore and thus speedup skey emulation.
Signed-off-by: NJanosch Frank <frankja@linux.ibm.com>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Acked-by: NFarhan Ali <alifm@linux.ibm.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>