mac80211: fix compilation error for old stable kernel version

mac80211: fix compilation error for old stable kernel version

Fix compilation error for old stable version caused by
genlmsg_multicast_allns backport fix pushed middle version.

Version 5.15 version 0-169, 6.1 version 0-115, 6.6 version 0-58 have the
old genlmsg_multicast_allns version with flags variable.

Compiling backport project with these version result in a compilation
error. To handle this, introduce a backport function for the affected
kernel version.

Link: https://github.com/openwrt/openwrt/pull/18373
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>