s390/perf_cpum_cf: Convert to use local64_try_cmpxchg()

Convert local64_cmpxchg() usages to local64_try_cmpxchg() in
order to generate slightly better code.

Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>