Add SCSI_PASSTHROUGH to zvols to enable UNMAP support

When exporting ZVOLs as SCSI LUNs, by default Windows will not
issue them UNMAP commands. This reduces storage efficiency in
many cases.

We add the SCSI_PASSTHROUGH flag to the zvol's device queue,
which lets the SCSI target logic know that it can handle SCSI
commands.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Gallagher <john.gallagher@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #8933

module/zfs/zvol.c

@@ -1876,6 +1876,10 @@ zvol_create_minor_impl(const char *name)
 #ifdef QUEUE_FLAG_ADD_RANDOM
 	blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, zv->zv_queue);
 #endif
+	/* This flag was introduced in kernel version 4.12. */
+#ifdef QUEUE_FLAG_SCSI_PASSTHROUGH
+	blk_queue_flag_set(QUEUE_FLAG_SCSI_PASSTHROUGH, zv->zv_queue);
+#endif
 
 	if (spa_writeable(dmu_objset_spa(os))) {
 		if (zil_replay_disable)