s390/cpum_cf: Fix endless loop in CF_DIAG event stop

Event CF_DIAG reads out complete counter sets using stcctm
instruction. This is done at event start time when the process
starts execution and at event stop time when the process is
removed from the CPU. During removal the difference of each
counter in the counter sets is calculated and saved as raw data
in the ring buffer. This works fine unless the number of counters
in a counter set is zero. This may happen for the extended counter
set. This set is machine specific and the size of the counter
set can be zero even when extended counter set is authorized for
read access.

This case is not handled. cfdiag_diffctr() checks authorization
of the extended counter set. If true the functions assumes
the extended counter set has been saved in a data buffer. However
this is not the case, cfdiag_getctrset() does not save a counter
set with counter set size of zero. This mismatch causes an endless
loop in the counter set readout during event stop handling.

The calculation of the difference of the counters in each counter
now verifies the size of the counter set is non-zero. A counter set
with size zero is skipped.

Fixes: a029a4ea

 ("s390/cpumf: Allow concurrent access for CPU Measurement Counter Facility")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>