rxrpc: Fix the rxrpc_connection attend queue handling

rxrpc: Fix the rxrpc_connection attend queue handling

The rxrpc_connection attend queue is never used because conn::attend_link
is never initialised and so is always NULL'd out and thus always appears to
be busy.  This requires the following fix:

 (1) Fix this the attend queue problem by initialising conn::attend_link.

And, consequently, two further fixes for things masked by the above bug:

 (2) Fix rxrpc_input_conn_event() to handle being invoked with a NULL
     sk_buff pointer - something that can now happen with the above change.

 (3) Fix the RXRPC_SKB_MARK_SERVICE_CONN_SECURED message to carry a pointer
     to the connection and a ref on it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
Fixes: f2cce89a074e ("rxrpc: Implement a mechanism to send an event notification to a connection")
Link: https://patch.msgid.link/20250203110307.7265-3-dhowells@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>