drbd: disambiguation, s/ERR_DISCARD/ERR_DISCARD_IMPOSSIBLE/

If for some reason (typically "split-brained" cluster manager)
drbd replica data has diverged, we can chose a victim,
and reconnect using "--discard-my-data", causing the victim
to become sync-target, fetching all changed blocks from the peer.

If we are Primary, we are potentially in use, and we refuse to
"roll back" changes to the data below the page cache and other users.

Rename the error symbol for this to ERR_DISCARD_IMPOSSIBLE.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_nl.c

@@ -1829,7 +1829,7 @@ _check_net_options(struct drbd_tconn *tconn, struct net_conf *old_conf, struct n
 				return ERR_STONITH_AND_PROT_A;
 		}
 		if (mdev->state.role == R_PRIMARY && new_conf->discard_my_data)
-			return ERR_DISCARD;
+			return ERR_DISCARD_IMPOSSIBLE;
 	}
 
 	if (new_conf->on_congestion != OC_BLOCK && new_conf->wire_protocol != DRBD_PROT_A)

include/linux/drbd.h

@@ -136,7 +136,7 @@ enum drbd_ret_code {
 	ERR_AUTH_ALG		= 120,
 	ERR_AUTH_ALG_ND		= 121,
 	ERR_NOMEM		= 122,
-	ERR_DISCARD		= 123,
+	ERR_DISCARD_IMPOSSIBLE	= 123,
 	ERR_DISK_CONFIGURED	= 124,
 	ERR_NET_CONFIGURED	= 125,
 	ERR_MANDATORY_TAG	= 126,