[S390] atomic ops: small cleanups

Couple of coding style fixes, replace __inline__ with inline and
remove #ifdef __KERNEL_- since the header file isn't exported.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>