s390/cpum_sf: Consistently use goto out for function exit

When the sampling buffer allocation fails in
__hw_perf_event_init(), jump to the end of the function
and return the result. This is consistent with the other
error handling and return conditions in this function.

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