s390/barrier: factor out bcr_serialize()

Factor out bcr_serialize() inline assembly function which describes
what the bcr instruction is used for.
Use bcr_serialize() like before in mb(), but also in upcoming changes.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>