userns: Allow the unprivileged users to mount mqueue fs

This patch allow the unprivileged user to mount mqueuefs in
user ns.

If two userns share the same ipcns,the files in mqueue fs
should be seen in both these two userns.

If the userns has its own ipcns,it has its own mqueue fs too.
ipcns has already done this job well.
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

ipc/mqueue.c

@@ -1383,6 +1383,7 @@ static struct file_system_type mqueue_fs_type = {
 	.name = "mqueue",
 	.mount = mqueue_mount,
 	.kill_sb = kill_litter_super,
+	.fs_flags = FS_USERNS_MOUNT,
 };
 
 int mq_init_ns(struct ipc_namespace *ns)