net/smc: Fix af_ops of child socket pointing to released memory

Child sockets may inherit the af_ops from the parent listen socket.
When the listen socket is released then the af_ops of the child socket
points to released memory.
Solve that by restoring the original af_ops for child sockets which
inherited the parent af_ops. And clear any inherited user_data of the
parent socket.

Fixes: 8270d9c2

 ("net/smc: Limit backlog connections")
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Reviewed-by: D. Wythe <alibuda@linux.alibaba.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>