rxrpc: Fix leak of connection from OOB challenge

rxrpc: Fix leak of connection from OOB challenge

Fix leak of connection object from OOB challenge queue when response is
provided by userspace.

Fixes: 5800b1cf3fd8 ("rxrpc: Allow CHALLENGEs to the passed to the app for a RESPONSE")
Link: https://sashiko.dev/#/patchset/20260609140911.838677-1-dhowells%40redhat.com
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
cc: stable@kernel.org
Link: https://patch.msgid.link/20260624163819.3017002-3-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>