batman-adv: bla: avoid CRC corruption due to parallel claim add

In the Linux kernel, the following vulnerability has been resolved:

batman-adv: bla: avoid CRC corruption due to parallel claim add

batadv_bla_add_claim() is used to add claims and modify the backbone of
claims for CLAIM frames from remote backbones and local packets. When it
handles a claim, it needs to either

* add the new claim's CRC to the backbone CRC
* remove the already existing claim's CRC from the old backbone and add it
to the new backbone

But when the "new" claim code was running in parallel to the "change
backbone" code, it can happen that the CRC was invalid because the
backbone_gw of the claim was changed twice in the "new" claim code path:

* CPU0 creates the claim for gateway A and publishes it in the claim
hash. The crc16 of the address has not yet been added to A's crc at
this point.

* CPU1 processes a claim frame of gateway B for the same client, finds
the just published claim, and performs the ownership change: it
switches the pointer to B, removes the crc16 from A's crc - which
never contained it - and adds it to B's crc.

* CPU0 continues behind the creation branch, unconditionally switches
the pointer back to A without compensating B's crc (its remove_crc
is false for the creation path), and finally adds the crc16 to A's
crc

The CRC is then wrong for both:

* claim belongs to A: but CRC is not part of backbone A's CRC
* claim doesn't belong to B: CRC is still part of backbone B's CRC

This wrong CRC is never recomputated from the stored claims. For local
backbone claims, this can also not recovered using syncs.

To avoid this, split the functionality in clear separate parts:

* new claim which always adds claim CRC to the backbone CRC (but never
changes the already set backbone_gw of the claim back)

* update of existing claim which automatically changes the backbone_gw
entry and only updates both backbone CRCs when there was an actual change