vhost-scsi: reduce flushes during endpoint clearing

vhost_scsi_flush will flush everything, so we can clear the backends then
flush, then destroy. We don't need to flush before each vq destruction
because after the flush we will have made sure there can be no new cmds
started and there are no running cmds.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Link: https://lore.kernel.org/r/20210525174733.6212-4-michael.christie@oracle.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>