rxrpc: rxperf: Fix uninitialised variable

Dan Carpenter sayeth[1]:

  The patch 75bfdbf2: "rxrpc: Implement an in-kernel rxperf server
  for testing purposes" from Nov 3, 2022, leads to the following Smatch
  static checker warning:

	net/rxrpc/rxperf.c:337 rxperf_deliver_to_call()
	error: uninitialized symbol 'ret'.

Fix this by initialising ret to 0.  The value is only used for tracing
purposes in the rxperf server.

Fixes: 75bfdbf2 ("rxrpc: Implement an in-kernel rxperf server for testing purposes")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: http://lists.infradead.org/pipermail/linux-afs/2022-December/006124.html

 [1]
Signed-off-by: David S. Miller <davem@davemloft.net>