rxrpc: rxperf: Fix missing decoding of terminal magic cookie

rxrpc: rxperf: Fix missing decoding of terminal magic cookie

The rxperf RPCs seem to have a magic cookie at the end of the request that
was failing to be taken account of by the unmarshalling of the request.
Fix the rxperf code to expect this.

Fixes: 75bfdbf2fca3 ("rxrpc: Implement an in-kernel rxperf server for testing purposes")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250218192250.296870-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>