KVM: s390: Cleanup usage of current->mm in set_guest_storage_key

In set_guest_storage_key, we really want to reference the mm struct given as
a parameter to the function. So replace the current->mm reference with the
mm struct passed in by the caller.

Signed-off-by: Jason J. Herne <jjherne@us.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>