zvol: Implement zvol threading as a Property

Currently, zvol threading can be switched through the zvol_request_sync
module parameter system-wide. By making it a zvol property, zvol
threading can be switched per zvol.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #15409

module/zfs/zvol.c

@@ -369,6 +369,20 @@ out:
 	return (SET_ERROR(error));
 }
 
+/*
+ * Update volthreading.
+ */
+int
+zvol_set_volthreading(const char *name, boolean_t value)
+{
+	zvol_state_t *zv = zvol_find_by_name(name, RW_NONE);
+	if (zv == NULL)
+		return (ENOENT);
+	zv->zv_threading = value;
+	mutex_exit(&zv->zv_state_lock);
+	return (0);
+}
+
 /*
  * Sanity check volume block size.
  */