ofproto: Fix double-unref of temporary rule when learning.

When ofproto_flow_mod_init() accepts a rule, it takes ownership of it and
either unrefs it on error or transfers ownership to the struct it
initializes on success, but ofproto_flow_mod_init_for_learn() was unref-ing
it a second time if it reported an error.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>

ofproto/ofproto-provider.h

@@ -1867,7 +1867,10 @@ struct rule_criteria {
 /* flow_mod with execution context. */
 struct ofproto_flow_mod {
     /* Allocated by 'init' phase, may be freed after 'start' phase, as these
-     * are not needed for 'revert' nor 'finish'. */
+     * are not needed for 'revert' nor 'finish'.
+     *
+     * This structure owns a reference to 'temp_rule' (if it is nonnull) that
+     * must be eventually be released with ofproto_rule_unref().  */
     struct rule *temp_rule;
     struct rule_criteria criteria;
     struct cls_conjunction *conjs;

ofproto/ofproto.c

@@ -4928,8 +4928,6 @@ ofproto_flow_mod_init_for_learn(struct ofproto *ofproto,
                                 struct ofproto_flow_mod *ofm)
     OVS_EXCLUDED(ofproto_mutex)
 {
-    enum ofperr error;
-
     /* Reject flow mods that do not look like they were generated by a learn
      * action. */
     if (fm->command != OFPFC_MODIFY_STRICT || fm->table_id == OFPTT_ALL
@@ -4970,13 +4968,7 @@ ofproto_flow_mod_init_for_learn(struct ofproto *ofproto,
         }
     }
 
-    /* Initialize ofproto_flow_mod for future use. */
-    error = ofproto_flow_mod_init(ofproto, ofm, fm, rule);
-    if (error) {
-        ofproto_rule_unref(rule);
-        return error;
-    }
-    return 0;
+    return ofproto_flow_mod_init(ofproto, ofm, fm, rule);
 }
 
 enum ofperr
@@ -7376,6 +7368,9 @@ ofproto_flow_mod_uninit(struct ofproto_flow_mod *ofm)
     }
 }
 
+/* Initializes 'ofm' with 'ofproto', 'fm', and 'rule'.  'rule' may be null, but
+ * if it is nonnull then the caller must own a reference to it, which on
+ * success is transferred to 'ofm' and on failure is unreffed. */
 static enum ofperr
 ofproto_flow_mod_init(struct ofproto *ofproto, struct ofproto_flow_mod *ofm,
                       const struct ofputil_flow_mod *fm, struct rule *rule)