batman-adv: Fix non-atomic bla_claim::backbone_gw access

The pointer batadv_bla_claim::backbone_gw can be changed at any time.
Therefore, access to it must be protected to ensure that two function
accessing the same backbone_gw are actually accessing the same. This is
especially important when the crc_lock is used or when the backbone_gw of a
claim is exchanged.

Not doing so leads to invalid memory access and/or reference leaks.

Fixes: 23721387 ("batman-adv: add basic bridge loop avoidance code")
Fixes: 5a1dd8a4 ("batman-adv: lock crc access in bridge loop avoidance")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>