CONFIG_NET_TCP_WRITE_BUFFERS=y lead to deadlock

Recently I've been testing FTPD operation on custom SAME70 based board with FileZilla FTP client.
I had `CONFIG_NET_TCP_WRITE_BUFFERS=y`.

I have a folder with 2 files that I try to upload from laptop to device. Upload of a folder fails.

FileZilla starts 2 FTP sessions in parallel one for each file upload so 2 instances of `ftpd_worker` are started.
The situation happens in 80% of my tests. The FTP client send `"STOR"` command for both files and by adding extra logs I see that both threads reach `ftpd.c:1908` code `ret = ftpd_response(session->cmd.sd, session->txtimeout, g_respfmt1, 150, ' ', "Opening data connection");` at the same time so `send` is called concurrently.

The logs before `ftpd_response` call are printed, but there are no logs after `ftpd_response` call. NSH "ps" shows that both threads are "waiting for semaphore", but most probably loop somewhere inside the `psock_tcp_send` trying to allocate `iob`.

Setting `CONFIG_NET_TCP_WRITE_BUFFERS=n` solves the issue and FTP server operates as expected.