rtnl_if_flags_to_linux() translated the usual IFF_* bits but dropped
FreeBSD's IFF_LOWER_UP (IFF_NETLINK_1). Chromium's AddressTrackerLinux
only treats a link as online when ifi_flags has UP|LOWER_UP|RUNNING; with
LOWER_UP missing, online_links stays empty, ConnectionType is
CONNECTION_NONE, and navigator.onLine is false even though TCP/HTTPS
work. Linux Chromium under the Linuxulator (e.g. www/linux-brave) then
shows a spurious Offline UI; sites that ignore navigator.onLine do not.
Native www/chromium is on a different notifier path.
Map IFF_LOWER_UP to Linux's IFF_LOWER_UP (1<<16) and define
LINUX_IFF_LOWER_UP alongside the existing LINUX_IFF_* constants.
PR: 297424