get rid of magic in proc_namespace.c

don't rely on proc_mounts->m being the first field; container_of()
is there for purpose.  No need to bother with ->private, while
we are at it - the same container_of will do nicely.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
index 5e289a7..5fe34c3 100644
--- a/fs/proc_namespace.c
+++ b/fs/proc_namespace.c
@@ -17,7 +17,7 @@
 
 static unsigned mounts_poll(struct file *file, poll_table *wait)
 {
-	struct proc_mounts *p = file->private_data;
+	struct proc_mounts *p = proc_mounts(file->private_data);
 	struct mnt_namespace *ns = p->ns;
 	unsigned res = POLLIN | POLLRDNORM;
 
@@ -121,7 +121,7 @@
 
 static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt)
 {
-	struct proc_mounts *p = m->private;
+	struct proc_mounts *p = proc_mounts(m);
 	struct mount *r = real_mount(mnt);
 	struct super_block *sb = mnt->mnt_sb;
 	struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
@@ -268,7 +268,6 @@
 	if (ret)
 		goto err_free;
 
-	p->m.private = p;
 	p->ns = ns;
 	p->root = root;
 	p->m.poll_event = ns->event;
@@ -288,7 +287,7 @@
 
 static int mounts_release(struct inode *inode, struct file *file)
 {
-	struct proc_mounts *p = file->private_data;
+	struct proc_mounts *p = proc_mounts(file->private_data);
 	path_put(&p->root);
 	put_mnt_ns(p->ns);
 	return seq_release(inode, file);