af_unix: handle idmapped mounts

When binding a non-abstract AF_UNIX socket it will gain a representation
in the filesystem. Enable the socket infrastructure to handle idmapped
mounts by passing down the user namespace of the mount the socket will
be created from. If the initial user namespace is passed nothing changes
so non-idmapped mounts will see identical behavior as before.

Link: https://lore.kernel.org/r/20210121131959.646623-18-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

net/unix/af_unix.c

@@ -996,8 +996,8 @@ static int unix_mknod(const char *sun_path, umode_t mode, struct path *res)
 	 */
 	err = security_path_mknod(&path, dentry, mode, 0);
 	if (!err) {
-		err = vfs_mknod(&init_user_ns, d_inode(path.dentry), dentry,
-				mode, 0);
+		err = vfs_mknod(mnt_user_ns(path.mnt), d_inode(path.dentry),
+				dentry, mode, 0);
 		if (!err) {
 			res->mnt = mntget(path.mnt);
 			res->dentry = dget(dentry);