xattr,net: support limited amount of extended attributes on sockfs sockets

xattr,net: support limited amount of extended attributes on sockfs sockets

Now that we've generalized the infrastructure for user.* xattrs make it
possible to set up to 128 user.* extended attributes on a sockfs inode
or up to 128kib. kernfs (cgroupfs) has the same limits and it has proven
to be quite sufficient for nearly all use-cases.

This will allow containers to label sockets and will e.g., be used by
systemd and Gnome to find various sockets in containers where
high-privilege or more complicated solutions aren't available.

Link: https://patch.msgid.link/20260216-work-xattr-socket-v1-10-c2efa4f74cb7@kernel.org
Acked-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>