Staging: batman-adv: don't lock while sending packets

As in other parts of batman-adv, we should not lock while sending a packet but
keep the lock held for as short as possible. Additionally, we should check
whether the interface is active, otherwise batman_if->net_dev might not be
available ...

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Acked-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>