firewall: change synflood_protect option name
The `syn_flood` option name is deprecated, `synflood_protect` should
be used instead. firewall3 and firewall4 both support this option since
a long time. LuCI already replaces the option name.
https://github.com/openwrt/luci/commit/0abcb39b623f0eff0fbcdfb99fca8f3224701e86
Suggested-by: rparge in OpenWrt forum
Link: https://github.com/openwrt/openwrt/pull/21642
Signed-off-by: Hauke Mehrtens <[email protected]>
Hauke Mehrtens