modules/kvs: Add fence checks

Previously, fences could be updated with new flags.  Instead,
require users to always specify the same nprocs and flags with
each fence request.  Return error if user does not.

Fixes #1305

src/modules/kvs/kvs.c

@@ -1727,6 +1727,12 @@ static void relayfence_request_cb (flux_t *h, flux_msg_handler_t *mh,
         }
     }
 
+    if (fence_get_flags (f) != flags
+        || fence_get_nprocs (f) != nprocs) {
+        errno = EINVAL;
+        goto error;
+    }
+
     if (fence_add_request_data (f, ops) < 0) {
         flux_log_error (h, "%s: fence_add_request_data", __FUNCTION__);
         goto error;
@@ -1792,8 +1798,12 @@ static void fence_request_cb (flux_t *h, flux_msg_handler_t *mh,
             goto error;
         }
     }
-    else
-        fence_set_flags (f, fence_get_flags (f) | flags);
+
+    if (fence_get_flags (f) != flags
+        || fence_get_nprocs (f) != nprocs) {
+        errno = EINVAL;
+        goto error;
+    }
 
     if (fence_add_request_copy (f, msg) < 0)
         goto error;