rxrpc: Fix oob challenge leak in cleanup after notification failure

rxrpc: Fix oob challenge leak in cleanup after notification failure

Fix rxrpc_notify_socket_oob() to return an indication of failure in the
event that it failed to queue a packet and fix rxrpc_post_challenge() to
clean up the connection ref in such an event.

Fixes: 5800b1cf3fd8 ("rxrpc: Allow CHALLENGEs to the passed to the app for a RESPONSE")
Link: https://sashiko.dev/#/patchset/20260616155749.2125907-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260624163819.3017002-8-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>