batman-adv: make batadv_test_bit() return 0 or 1 only

On some architectures test_bit() can return other values than 0 or 1:

With a generic x86 OpenWrt image in a kvm setup (batadv_)test_bit()
frequently returns -1 for me, leading to batadv_iv_ogm_update_seqnos()
wrongly signaling a protected seqno window.

This patch tries to fix this issue by making batadv_test_bit() return 0
or 1 only.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Acked-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: David S. Miller <davem@davemloft.net>