storage: Error out if the target is already mounted for netfs pool

mnt_fsname can not be the same, as we check the duplicate pool
sources earlier before, means it can't be the same pool, moreover,
a pool can't be started if it's already active anyway. So no reason
to act as success.
(cherry picked from commit 81165294)

src/storage/storage_backend_fs.c

@@ -407,10 +407,10 @@ virStorageBackendFileSystemMount(virStoragePoolObjPtr pool) {
 
     /* Short-circuit if already mounted */
     if ((ret = virStorageBackendFileSystemIsMounted(pool)) != 0) {
-        if (ret < 0)
+        virStorageReportError(VIR_ERR_OPERATION_INVALID,
+                              _("Target '%s' is already mounted"),
+                              pool->def->target.path);
         return -1;
-        else
-            return 0;
     }
 
     if (pool->def->type == VIR_STORAGE_POOL_NETFS) {