bury struct proc_ns in fs/proc

a) make get_proc_ns() return a pointer to struct ns_common
b) mirror ns_ops in dentry->d_fsdata of ns dentries, so that
is_mnt_ns_file() could get away with fewer dereferences.

That way struct proc_ns becomes invisible outside of fs/proc/*.c

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 87c3722..49746c8 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -222,7 +222,6 @@
 {
 	struct task_struct *tsk = current;
 	struct nsproxy *new_nsproxy;
-	struct proc_ns *ei;
 	struct file *file;
 	struct ns_common *ns;
 	int err;
@@ -232,8 +231,7 @@
 		return PTR_ERR(file);
 
 	err = -EINVAL;
-	ei = get_proc_ns(file_inode(file));
-	ns = ei->ns;
+	ns = get_proc_ns(file_inode(file));
 	if (nstype && (ns->ops->type != nstype))
 		goto out;