scsi: scsi_debug: Add poll mode deferred completions to statistics

Currently commands completed via poll mode are not included in the
statistics gathering for deferred completions and missed CPUs.

Poll mode completions should be treated the same as other deferred
completion types, so add poll mode completions to the statistics.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Link: https://lore.kernel.org/r/20230313093114.1498305-12-john.g.garry@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>