virtio/s390: get rid of open-coded kvm hypercall

virtio/s390: get rid of open-coded kvm hypercall

do_kvm_notify() and __do_kvm_notify() are an (exact) open-coded variant
of kvm_hypercall3(). Therefore simply make use of kvm_hypercall3(),
and get rid of duplicated code.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Link: https://lore.kernel.org/r/20210621144522.1304273-1-hca@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>