s390/facility: decompose test_facility()

The patch decomposes the function test_facility() into its API
test_facility() and its implementation __test_facility(). This
allows to reuse the implementation with a different API.

Patch is used to prepare checkin of SIE satellite code.

Signed-off-by: Michael Mueller <mimu@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>