batman-adv: Don't always delete softif when last slave was removed

batman-adv has an unusual way to manage softinterfaces. These will be created
automatically when a user writes to the batman-adv/mesh_iface file in sysfs and
removed when no slave device exists anymore.

This behaviour cannot be changed without breaking compatibility with existing
code. Instead other interfaces should be able to slightly reduce this behaviour
and provide a more common reaction to a removal of a slave interface.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>