batman: fix a bogus warning from batadv_is_on_batman_iface()

batman tries to search dev->iflink to check if it's a batman interface,
but ->iflink could be 0, which is not a valid ifindex. It should just
avoid iflink == 0 case.

Reported-by: Jet Chen <jet.chen@intel.com>
Tested-by: Jet Chen <jet.chen@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Antonio Quartulli <antonio@open-mesh.com>
Cc: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>