userns: Rename id_map_mutex to userns_state_mutex

commit f0d62aec upstream.

Generalize id_map_mutex so it can be used for more state of a user namespace.

Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>