s390/cio: Introduce generic synchronous CHSC IOCTL

This patch adds a new ioctl CHSC_START_SYNC that allows to
execute any synchronous CHSC that is provided by user space.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>