rxrpc: Fix RESPONSE packet verification to extract skb to a linear buffer

rxrpc: Fix RESPONSE packet verification to extract skb to a linear buffer

This improves the fix for CVE-2026-43500.

Fix the verification of RESPONSE packets to avoid the problem of
overwriting a RESPONSE packet sent via splice to a local address by
extracting the contents of the UDP packet into a kmalloc'd linear buffer
rather than decrypting the data in place in the sk_buff (which may corrupt
the original buffer).

Fixes: 24481a7f5733 ("rxrpc: Fix conn-level packet handling to unshare RESPONSE packets")
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Closes: https://lore.kernel.org/r/afKV2zGR6rrelPC7@v4bel/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: Jiayuan Chen <jiayuan.chen@linux.dev>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
Link: https://patch.msgid.link/20260515230516.2718212-4-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>