s390/ism: Introduce struct ism_dmb

Conceptually, a DMB is a structure that belongs to ISM devices. However,
SMC currently 'owns' this structure. So future exploiters of ISM devices
would be forced to include SMC headers to work - which is just weird.
Therefore, we switch ISM to struct ism_dmb, introduce a new public header
with the definition (will be populated with further API calls later on),
and, add a thin wrapper to please SMC. Since structs smcd_dmb and ism_dmb
are identical, we can simply convert between the two for now.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Signed-off-by: Jan Karcher <jaka@linux.ibm.com>
Signed-off-by: Wenjia Zhang <wenjia@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>