s390: drop smp_mb in vdso_init

The initial s390 vdso code is heavily influenced by the powerpc version
which does have a smp_wmb in vdso_init right before the vdso_ready=1
assignment. s390 has no need for that.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <1452010645-25380-1-git-send-email-mst@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>