mp: Fix game password being sent under the wrong parent node (bug #23015)

The password was sent under the client message's root node, rather than
the [create_game] subnode where the MP server expects to find the
password. This caused games created by affected clients to always lack a
join password.

This bug was presumably introduced by commit
13e22fd6f153d10f64888db1655a0e8add9a871f (1.11.6+dev).