[S390] ccwreq: add ability to use all paths

Change the ccwrequest infrastructure to use more than one channel
path per start I/O. A flag "singlepath" is added to struct
ccw_request - if set, the old behavior is used. This flag is set
for all exploiters of the ccwrequest infrastructure - so there
is no functional change through this patch.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>