networking: use OPENVPN_ETH_ALEN instead of ETH_ALEN

networking: use OPENVPN_ETH_ALEN instead of ETH_ALEN

With the introduction of SITNL two occurrences of ETH_ALEN
were introduced in the code. This define exists on Linux in
the linux/if_ether.h header, however the latter is included
in linux/if_tun.h which may or may not exist (depending on
how old your system is).

If the system lacks if_tun.h then the compilation fails because
ETH_ALEN cannot be found.

For this reason, swap ETH_ALEN with OPENVPN_ETH_ALEN which is
already used in other places of the code.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220223080628.4773-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23882.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>