Reschedule processes on -ERESTARTSYS

On the single core machine the system may hang when the
spa_namespare_lock acquisition fails in the zvol_first_open
function. It returns -ERESTARTSYS error what causes the
endless loop in __blkdev_get function.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Arkadiusz Bubała <arkadiusz.bubala@open-e.com>
Closes #6283 
Closes #6312

module/zfs/zvol.c

@@ -1358,6 +1358,8 @@ out_mutex:
 	mutex_exit(&zv->zv_state_lock);
 	if (drop_suspend)
 		rw_exit(&zv->zv_suspend_lock);
+	if (error == -ERESTARTSYS)
+		schedule();
 
 	return (SET_ERROR(error));
 }