s390/cpum_cf: Handle EBUSY return code from CPU counter facility reservation

Rservation of the CPU Measurement Counter facility may fail if
it is already in use by the cf_diag device driver.
This is indicated by a non zero return code (-EBUSY).
However this return code is ignored and the counter facility
may be used in parallel by different device drivers.

Handle the failing reservation and return an error to the
caller.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>