s390/cpum_cf: prepare for in-kernel counter measurements

Prepare the counter facility support to be used by other in-kernel
users.  The first step introduces the __kernel_cpumcf_begin() and
__kernel_cpumcf_end() functions to reserve the counter facility
for doing measurements and to release after the measurements are
done.

Signed-off-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>