virtiofsd: Error on bad socket group name

Make the '--socket-group=' option fail if the group name is unknown:

./tools/virtiofsd/virtiofsd .... --socket-group=zaphod
vhost socket: unable to find group 'zaphod'

Reported-by: Xiaoling Gao <xiagao@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20211014122554.34599-1-dgilbert@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/fuse_virtio.c

@@ -988,6 +988,13 @@ static int fv_create_listen_socket(struct fuse_session *se)
                          "vhost socket failed to set group to %s (%d): %m\n",
                          se->vu_socket_group, g->gr_gid);
             }
+        } else {
+            fuse_log(FUSE_LOG_ERR,
+                     "vhost socket: unable to find group '%s'\n",
+                     se->vu_socket_group);
+            close(listen_sock);
+            umask(old_umask);
+            return -1;
         }
     }
     umask(old_umask);