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/net/core/net_namespace.c b/net/core/net_namespace.c
index 4d4acaf..ce780c7 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -337,7 +337,6 @@
 
 struct net *get_net_ns_by_fd(int fd)
 {
-	struct proc_ns *ei;
 	struct file *file;
 	struct ns_common *ns;
 	struct net *net;
@@ -346,8 +345,7 @@
 	if (IS_ERR(file))
 		return ERR_CAST(file);
 
-	ei = get_proc_ns(file_inode(file));
-	ns = ei->ns;
+	ns = get_proc_ns(file_inode(file));
 	if (ns->ops == &netns_operations)
 		net = get_net(container_of(ns, struct net, ns));
 	else