net: document NETDEV_CHANGENAME as ops locked

net: document NETDEV_CHANGENAME as ops locked

NETDEV_CHANGENAME is only emitted from netif_change_name().
netif_change_name() has two callers both of which hold netdev_lock_ops()
around the call site:
 - dev_change_name()
 - do_setlink()

Document NETDEV_CHANGENAME as always ops locked.

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260603012840.2254293-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>