[S390] Expose a constant for the number of words representing the CRs

We need to use this value in the checkpoint/restart code and would like to
have a constant instead of a magic '3'.

Cc: linux-s390@vger.kernel.org
Signed-off-by: Dan Smith <danms@us.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>