batman: fix a bogus warning from batadv_is_on_batman_iface()

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 <[email protected]>
Tested-by: Jet Chen <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Steffen Klassert <[email protected]>
Cc: Antonio Quartulli <[email protected]>
Cc: Marek Lindner <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>