scsi: core: add scsi_host_(block,unblock) helper function

Add helper functions to call scsi_internal_device_block()/
scsi_internal_device_unblock() for all attached devices on a SCSI host.

Link: https://lore.kernel.org/r/20200228075318.91255-9-hare@suse.de


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>