drbd: Refuse to change network options online when...

* the peer does not speak protocol_version 100 and the
  user wants to change one of:
    - wire_protocol
    - two_primaries
    - integrity_alg

* the user wants to remove the allow_two_primaries flag
  when there are two primaries

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
diff --git a/include/linux/drbd.h b/include/linux/drbd.h
index 05063e6..679e811 100644
--- a/include/linux/drbd.h
+++ b/include/linux/drbd.h
@@ -161,6 +161,7 @@
 	ERR_MINOR_EXISTS	= 161,
 	ERR_INVALID_REQUEST	= 162,
 	ERR_NEED_APV_100	= 163,
+	ERR_NEED_ALLOW_TWO_PRI  = 164,
 
 	/* insert new ones above this line */
 	AFTER_LAST_ERR_CODE