proc: fix dentry/inode overinstantiating under /proc/${pid}/net

When a process exits, /proc/${pid}, and /proc/${pid}/net dentries are
flushed.  However some leaf dentries like /proc/${pid}/net/arp_cache
aren't.  That's because respective PDEs have proc_misc_d_revalidate() hook
which returns 1 and leaves dentries/inodes in the LRU.

Force revalidation/lookup on everything under /proc/${pid}/net by
inheriting proc_net_dentry_ops.

[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/YjdVHgildbWO7diJ@localhost.localdomain
Fixes: c6c75ded ("proc: fix lookup in /proc/net subdirectories after setns(2)")
Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Reported-by: Nhui li <juanfengpy@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>

fs/proc/generic.c

@@ -448,6 +448,9 @@ static struct proc_dir_entry *__proc_create(struct proc_dir_entry **parent,
 	proc_set_user(ent, (*parent)->uid, (*parent)->gid);
 
 	ent->proc_dops = &proc_misc_dentry_ops;
+	/* Revalidate everything under /proc/${pid}/net */
+	if ((*parent)->proc_dops == &proc_net_dentry_ops)
+		pde_force_lookup(ent);
 
 out:
 	return ent;

fs/proc/proc_net.c

@@ -376,6 +376,9 @@ static __net_init int proc_net_ns_init(struct net *net)
 
 	proc_set_user(netd, uid, gid);
 
+	/* Seed dentry revalidation for /proc/${pid}/net */
+	pde_force_lookup(netd);
+
 	err = -EEXIST;
 	net_statd = proc_net_mkdir(net, "stat", netd);
 	if (!net_statd)