scsi: fnic: Use scsi_host_busy_iter() to traverse commands

Use scsi_host_busy_iter() to traverse commands instead of hand-crafted
routines walking the command list.

Link: https://lore.kernel.org/r/20210429122517.39659-3-hare@suse.de
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: hreinecke