scsi: target: iblock: Fix smp_processor_id() BUG messages

This has us use raw_smp_processor_id() in iblock's plug_device callout.
smp_processor_id() is not needed here, because we are running from a per
CPU work item that is also queued to run on a worker thread that is
normally bound to a specific CPU. If the worker thread did end up switching
CPUs then it's handled the same way we handle when the work got moved to a
different CPU's worker thread, where we will just end up sending I/O from
the new CPU.

Link: https://lore.kernel.org/r/20210519222640.5153-1-michael.christie@oracle.com
Fixes: 415ccd98

 ("scsi: target: iblock: Add backend plug/unplug callouts")
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>