batman-adv: compat: Add atomic mc_forwarding support for stable kernels

batman-adv: compat: Add atomic mc_forwarding support for stable kernels

Eric Dumazet changed the mc_forwarding in commit 145c7a793838 ("ipv6: make
mc_forwarding atomic") the type of mc_forwarding from __s32 to an atomic_t.
This patch was then ported to various stable kernels.

This code change caused a FTBFS when in batman-adv when
CONFIG_BATMAN_ADV_MCAST was activated. To work around this problem, provide
a version for kernels with __s32 mc_forwarding and a version for kernel
with atomic_t mc_forwarding.

Fixes: #850
Reported-by: Huangbin Zhan <zhanhb88@gmail.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>