s390/cpu_mf: move struct cpu_cf_events and per-CPU variable to header file

Make the struct cpu_cf_events and the respective per-CPU variable available
to in-kernel users.  Access to this per-CPU variable shall be done between
the calls to __kernel_cpumcf_begin() and __kernel_cpumcf_end().

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