ipc: enable is_ns_init_id() assertions
The ipc namespace may call put_ipc_ns() and get_ipc_ns() before it is
added to the namespace tree. Assign the id early like we do for a some
other namespaces.
Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-11-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>