proc: use generic setattr() for /proc/$PID/net

All other files in /proc/$PID/ use proc_setattr().

Not using it allows the usage of chmod() on /proc/$PID/net, even on
other processes owned by the same user.
The same would probably also be true for other attributes to be changed.

As this technically represents an ABI change it is not marked for
stable so any unlikely regressions are caught during a full release cycle.

Fixes: e9720acd ("[NET]: Make /proc/net a symlink on /proc/self/net (v3)")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/lkml/d0d111ef-edae-4760-83fb-36db84278da1@t-8ch.de/
Fixes: b4844fa0 ("selftests/nolibc: implement a few tests for various syscalls")
Tested-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Message-Id: <20230624-proc-net-setattr-v1-2-73176812adee@weissschuh.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/proc/proc_net.c

@@ -321,6 +321,7 @@ static int proc_tgid_net_getattr(struct mnt_idmap *idmap,
 const struct inode_operations proc_net_inode_operations = {
 	.lookup		= proc_tgid_net_lookup,
 	.getattr	= proc_tgid_net_getattr,
+	.setattr        = proc_setattr,
 };
 
 static int proc_tgid_net_readdir(struct file *file, struct dir_context *ctx)