nvmet-tcp: fixup hang in nvmet_tcp_listen_data_ready()

In the Linux kernel, the following vulnerability has been resolved:

nvmet-tcp: fixup hang in nvmet_tcp_listen_data_ready()

When the socket is closed while in TCP_LISTEN a callback is run to

flush all outstanding packets, which in turns calls

nvmet_tcp_listen_data_ready() with the sk_callback_lock held.

So we need to check if we are in TCP_LISTEN before attempting

to get the sk_callback_lock() to avoid a deadlock.