batman-adv: frag: disallow unicast fragment in fragment

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

batman-adv: frag: disallow unicast fragment in fragment

batadv_frag_skb_buffer() is called by batadv_batman_skb_recv() when a

BATADV_UNICAST_FRAG packet is received. Once all fragments are collected

and the packet is reassembled, batadv_recv_frag_packet() calls

batadv_batman_skb_recv() again to process the defragmented payload.

A malicious sender can craft a BATADV_UNICAST_FRAG packet whose reassembled

payload is itself a BATADV_UNICAST_FRAG packet (matryoshka-style nesting).

Each nesting level recurses through batadv_batman_skb_recv() without bound,

growing the kernel stack until it is exhausted.

Since refragmentation or fragments in fragments are not actually allowed,

discard all packets which are still BATADV_UNICAST_FRAG packets after the

defragmentation process.