s390/cpum_sf: Rework attribute definition for diagnostic sampling

Previously, the attribute entry for diagnostic sampling was added
if authorized.  Otherwise, the array of struct attribute contains
two NULL values.

Change this logic and reserve space for the attribute for diagnostic
sampling. If diagnostic sampling is authorized, add an entry in the
respective position in the array of struct attribute.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Suggested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>