ax25: properly unshare skbs in ax25_kiss_rcv()

Bernard Pidoux reported a regression apparently caused by commit
c353e8983e0d ("net: introduce per netns packet chains").

skb->dev becomes NULL and we crash in __netif_receive_skb_core().

Before above commit, different kind of bugs or corruptions could happen
without a major crash.

But the root cause is that ax25_kiss_rcv() can queue/mangle input skb
without checking if this skb is shared or not.

Many thanks to Bernard Pidoux for his help, diagnosis and tests.

We had a similar issue years ago fixed with commit 7aaed57c5c28
("phonet: properly unshare skbs in phonet_rcv()").

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Bernard Pidoux <f6bvp@free.fr>
Closes: https://lore.kernel.org/netdev/1713f383-c538-4918-bc64-13b3288cd542@free.fr/
Tested-by: Bernard Pidoux <f6bvp@free.fr>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Joerg Reuter <jreuter@yaina.de>
Cc: David Ranch <dranch@trinnet.net>
Cc: Folkert van Heusden <folkert@vanheusden.com>
Reviewed-by: Dan Cross <crossd@gmail.com>
Link: https://patch.msgid.link/20250902124642.212705-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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