scsi: fc: check for rport presence in fc_block_scsi_eh

Coverity-scan recently found a possible NULL pointer dereference in
fc_block_scsi_eh() as starget_to_rport() either returns the rport for
the startget or NULL.

While it is rather unlikely to have fc_block_scsi_eh() called without an
rport associated it's a good idea to catch potential misuses of the API
gracefully.
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_transport_fc.c

@@ -3320,6 +3320,9 @@ int fc_block_scsi_eh(struct scsi_cmnd *cmnd)
 {
 	struct fc_rport *rport = starget_to_rport(scsi_target(cmnd->device));
 
+	if (WARN_ON_ONCE(!rport))
+		return FAST_IO_FAIL;
+
 	return fc_block_rport(rport);
 }
 EXPORT_SYMBOL(fc_block_scsi_eh);