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/mount.h b/fs/mount.h
index 05a2a11..4f291f9 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -74,10 +74,12 @@
 }
 
 struct proc_mounts {
-	struct seq_file m; /* must be the first element */
+	struct seq_file m;
 	struct mnt_namespace *ns;
 	struct path root;
 	int (*show)(struct seq_file *, struct vfsmount *);
 };
 
+#define proc_mounts(p) (container_of((p), struct proc_mounts, m))
+
 extern const struct seq_operations mounts_op;