s390/mm: use shared variables for sysctl range check

Since commit eec4844f

 ("proc/sysctl: add shared variables for range
check") special shared variables are available for sysctl range check.
Reuse them for /proc/sys/vm/allocate_pgste proc handler.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>