Fix spa config generate memory leak in spa_load_best function

When spa retry load succeeds and spa recovery is requested it may
leak in spa_load_best function.  Always free the generated config
when it is not assigned to the spa.

Signed-off-by: cao.xuewen <cao.xuewen@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4940

module/zfs/spa.c

@@ -3095,6 +3095,8 @@ spa_load_best(spa_t *spa, spa_load_state_t state, int mosconfig,
 
 	if (config && (rewind_error || state != SPA_LOAD_RECOVER))
 		spa_config_set(spa, config);
+	else
+		nvlist_free(config);
 
 	if (state == SPA_LOAD_RECOVER) {
 		ASSERT3P(loadinfo, ==, NULL);