s390/cpum_sf: Set bit PMU_F_ENABLED enabled after lpp() invocation

Set PMU enabled bit after lpp() call. This ensures the proper
task PID is loaded by the llp instruction into Program Parameter
register from where it is copied to each sampling data buffer
(SDB) sample entry after the sampling was enabled.

The barrier() instruction is not needed as cpumsf_pmu_enable()
changes a CPU specific variable. Only the CPU that task is
running on changes structure members.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>